|FISH
Location Diet|G|H|I|J|K|L|M
----------------------------------------------------------------
=Black-footed albatross= _(Diomedea nigripes)_
No. Pacific (Palmer 1962) x x
Aleutian Islands (Cottam and Knappen 1939) x x
California (Miller 1936, 1940) x x x
=Laysan albatross= _(D. immutabilis)_
No. Pacific (Palmer 1962; Bartsch 1922;
Fisher 1904)
=Northern fulmar= _(Fulmarus glacialis)_
Pribilof Islands (Preble and McAtee 1923)
Alaska (Gabrielson and Lincoln 1959) x
Oregon (Gabrielson and Jewett 1940)
No. Atlantic (Hartley and Fisher 1936;
Einarsson 1945; Fisher 1952) x
=Flesh-footed shearwater= _(Puffinus carneipes)_
Australia (Oliver 1955; Serventy et al. 1971) x
=Pink-footed shearwater= _(P. creatopus)_
California (Murphy 1936; Ainley,
personal observation) x
E. Pacific (Cottam and Knappen 1939) x
=Buller's shearwater= _(P. bulleri)_
SW Pacific (Falla 1934; Serventy et al. 1971) x
Peru (Murphy 1936)
=Sooty shearwater= _(P. griseus)_
Aleutian Islands (Sanger, personal
observation) x
British Columbia (Martin 1942; Sealy 1973_a_) x x
Oregon (Gabrielson and Jewett 1940)
California (Ainley, personal observation) x
Peru (Murphy 1936) x
SW Pacific (Oliver 1955; Serventy et al. 1971) x
=Short-tailed shearwater= _(P. tenuirostris)_
Bristol Bay (Bartonek, personal communication)
Alaska (Cottam and Knappen 1939) o
No. Pacific (Palmer 1962; Kuroda 1955) x
Australia (Serventy et al. 1971) x
Bass Strait (Sheard 1953)
=Mottled petrel= _(Pterodroma inexpectata)_
Pacific Ocean (Imber 1973) x
E. No. Pacific (Kuroda 1955) x
=Fork-tailed storm-petrel= _(Oceanodroma furcata)_
Pribilof Islands (Preble and McAtee 1923) x
SE Alaska (Heath 1915)
British Columbia (Martin 1942) x
California (Ainley, personal observation) x
=Leach's storm-petrel= _(O. leucorhoa)_
SE Alaska (Heath 1915)
California (PRBO, unpublished data) x x
So. California (Palmer 1962) x
No. Atlantic[31] (Palmer 1962) x x
----------------------------------------------------------------
Table 9. _Diets of gulls in different localities (x = major prey,
o = minor prey, and * = incidental prey species)._
Key:
CRUSTACEAN
A: Euphausiid
B: Barnacle
C: Decapod
MOLLUSC
D: Shell fish
E: Cephalopod
FISH
F: "Fish"
G: _Clupea_
H: _Engraulis_
I: _Osmerus_
J: _Porichihys_
K: _Otaphidium_
L: _Mallotus_
M: _Borrogadus_
N: _Microgadus_
O: _Gadus_
P: _Lycodes_
Q: _Sebastes_
R: _Myxocephalus_
S: _Genyonemus_
T: Embiotocid
U: _Ammodytes_
BIRD
V: Eggs
X: Chicks
Y: Adults
|CRUSTACEAN
| |POLYCHAETE
| | |MOLLUSC
| | | |ECHINODERM
| | | | |COELENTERATE
| | | | | |FISH
Diet | | | | | |
Location |A|B|C| |D|E| | |F|G|H|I|
---------------------------------------------------------------------------
=Glaucous gull= _(Larus hyperboreus)_
St. Lawrence Island (Fay and Cade 1959)
Chukchi Sea (Swartz 1966) x x x
Pribilof Islands (Preble and McAtee 1923) x x x x
Vancouver Island (Munro and Clemens 1931) x
=Glaucous-winged gull= _(L. glaucescens)_[32]
Pribilof Islands (Preble and McAtee 1923) x x x
Alaska (Bent 1921)
No. Pacific (Sanger 1973) x * *
Mandarte Island (Ward 1973) x x x
Vancouver Island (Munro and Clemens 1931; x x
Robertson, unpublished data)
=Western gull= _(L. occidentalis)_[33]
Farallon Islands (PRBO, unpublished data) x x o * x * * x
=Herring gull= _(L. argentatus)_
No. Atlantic (Zelikman 1961) x
E. No. America (Bent 1946; Ainley, x x x x x
personal observation)
Vancouver Island (Munro and Clemens 1931) x x
=Mew gull= _(L. canus)_
Alaska (Bent 1921) x
Vancouver Island (Munro and Clemens 1931) x x
=Heermann's gull= _(L. heermanni)_
California (Bent 1921) x x x
=Bonaparte's gull= _(L. philadelphia)_
E. No. America (Bent 1921) x x x
=Black-legged kittiwake= _(Rissa tridactyla)_[34]
Chukchi Sea (Swartz 1966) o * o
Pribilof Islands (Preble and McAtee 1923) o x
Alaska (Bent 1921) x
Cook Inlet[35] (Snarski, personal o o o
communication)
No. Atlantic (Hartley and Fisher x
1936; Zelikman 1961)
=Red-legged kittiwake= _(R. breuirostris)_
Pribilof Islands (Preble and McAtee 1923) x x x
=Sabine's gull= _(Xenia sabini)_
Pt. Barrow (Banner 1954) x
---------------------------------------------------------------------------
Location Diet |J|K|L|M|N|O|P|Q|R|S|T|U|
-----------------------------------------------------------------------------
=Glaucous gull= _(Larus hyperboreus)_
St. Lawrence Island (Fay and Cade 1959)
Chukchi Sea (Swartz 1966) x x
Pribilof Islands (Preble and McAtee 1923)
Vancouver Island (Munro and Clemens 1931)
=Glaucous-winged gull= _(L. glaucescens)_[32]
Pribilof Islands (Preble and McAtee 1923)
Alaska (Bent 1921)
No. Pacific (Sanger 1973)
Mandarte Island (Ward 1973) x
Vancouver Island (Munro and Clemens 1931;
Robertson, unpublished data)
=Western gull= _(L. occidentalis)_[33]
Farallon Islands (PRBO, unpublished data) x o o o x o o
=Herring gull= _(L. argentatus)_
No. Atlantic (Zelikman 1961)
E. No. America (Bent 1946; Ainley,
personal observation)
Vancouver Island (Munro and Clemens 1931)
=Mew gull= _(L. canus)_
Alaska (Bent 1921) x
Vancouver Island (Munro and Clemens 1931)
=Heermann's gull= _(L. heermanni)_
California (Bent 1921)
=Bonaparte's gull= _(L. philadelphia)_
E. No. America (Bent 1921)
=Black-legged kittiwake= _(Rissa tridactyla)_[34]
Chukchi Sea (Swartz 1966) o x o o x
Pribilof Islands (Preble and McAtee 1923)
Alaska (Bent 1921)
Cook Inlet[35] (Snarski, personal x o x
communication)
No. Atlantic (Hartley and Fisher
1936; Zelikman 1961)
=Red-legged kittiwake= _(R. breuirostris)_
Pribilof Islands (Preble and McAtee 1923)
=Sabine's gull= _(Xenia sabini)_
Pt. Barrow (Banner 1954)
-----------------------------------------------------------------------------
Diet |CARRION-OFFAL
| |BIRD
| | |FISH EGGS
Location | |V|X|Y|
----------------------------------------------------------------------
=Glaucous gull= _(Larus hyperboreus)_
St. Lawrence Island (Fay and Cade 1959) x x x
Chukchi Sea (Swartz 1966) x x
Pribilof Islands (Preble and McAtee 1923) x
Vancouver Island (Munro and Clemens 1931) x
=Glaucous-winged gull= _(L. glaucescens)_[32]
Pribilof Islands (Preble and McAtee 1923) x x x x
Alaska (Bent 1921) x
No. Pacific (Sanger 1973)
Mandarte Island (Ward 1973) x
Vancouver Island (Munro and Clemens 1931; x
Robertson, unpublished data)
=Western gull= _(L. occidentalis)_[33]
Farallon Islands (PRBO, unpublished data) x o o o
=Herring gull= _(L. argentatus)_
No. Atlantic (Zelikman 1961)
E. No. America (Bent 1946; Ainley, x * o x
personal observation)
Vancouver Island (Munro and Clemens 1931) x
=Mew gull= _(L. canus)_
Alaska (Bent 1921) x x
Vancouver Island (Munro and Clemens 1931) x
=Heermann's gull= _(L. heermanni)_
California (Bent 1921) x
=Bonaparte's gull= _(L. philadelphia)_
E. No. America (Bent 1921)
=Black-legged kittiwake= _(Rissa tridactyla)_[34]
Chukchi Sea (Swartz 1966)
Pribilof Islands (Preble and McAtee 1923) o
Alaska (Bent 1921) o
Cook Inlet[35] (Snarski, personal
communication)
No. Atlantic (Hartley and Fisher
1936; Zelikman 1961)
=Red-legged kittiwake= _(R. breuirostris)_
Pribilof Islands (Preble and McAtee 1923)
=Sabine's gull= _(Xenia sabini)_
Pt. Barrow (Banner 1954)
----------------------------------------------------------------------
=Table 10=. _Diets of terns in different localities (x = major prey
species)._
------------------------------------------------------------------------
CRUSTACEAN FISH
------------------------------------------------------------------------
|"Crustacean"|_Mallotus_
| |Euphausiid| |Cottid
Diet | | |Amphipod| | |_Ammodytes_
Location | | | | | | larvae
------------------------------------------------------------------------
=Arctic tern= _(Sterna paradisaea)_
Pribilof Islands (Preble and McAtee 1923) x x
Alaska (Bent 1921) x x x
No. Atlantic (Hartley and Fisher 1936) x
=Common tern= _(S. hirundo)_
E. No. America (Bent 1921) x x
------------------------------------------------------------------------
The oceanic habitat includes waters of the photic zone overlying the
deep ocean and continental slopes beyond the continental or insular
shelves. The Bering Sea and central subarctic domains are largely made
up of oceanic habitat. The other three domains include both inshore and
offshore neritic as well as some oceanic habitat. The boundary between
the inshore and offshore neritic has yet to be defined in terms of bird
life, but it lies at that line beyond which the bottom is too deep for
a diving bird to exploit. A depth contour thus defines the boundary. In
the antarctic South Pacific, emperor penguins _(Aptenodytes fosteri)_
dive to depths of 275 m, but so far as is known, no comparable bird
exists in the North Pacific. Some marine ducks and loons reportedly
dive to 50-60 m (Kooyman 1974). The inshore-offshore neritic boundary
for seabirds may lie near the 70-m depth contour.
Food resource partitioning by seabirds in the five oceanographic
domains are shown in Tables 11-15. Within each domain, the common
and usual members of the seabird community are listed, and the major
and minor categories in each of their diets are shown (on the basis
of available literature, Tables 1-10). The categories are grouped
further, and rather tenuously, according to the trophic level at which
a bird is presumably feeding: I = herbivore, II = secondary carnivore,
III = tertiary carnivore, IV = final carnivore, and Sc = scavengers
(carnivorous) feeding at many levels. Birds at level I feed on large
algae and seed plants and are not directly part of the same food webs
involving other species. These food webs originate with phytoplankton
(Fig. 2). So far as is known, no bird feeds on phytoplankton and few,
if any, feed on microzooplankton; hence birds do not generally feed as
primary carnivores. An exception at times might be the least auklet
_(Aethia pusilla)_ when it feeds on small copepods (see Bédard 1969_b_).
The above groupings are "tenuous" because prey in each category may
represent more than one trophic level, and a single prey species could
occur at one level one day or place and at another level the next day
or place, depending upon what it happened to be eating. This is shown
in Fig. 2, where the parakeet auklet _(Cyclorrhynchus psittaculus)_
can occur in the food web at different levels, depending both on
the prey it is eating and on what its prey is eating. Even without
this complication, many seabirds feed at more than one level in the
food web. For instance, murres eating euphausiids would be feeding
at a different level than murres feeding on larger fish. It might be
"safer" to regard prey organisms in level II as macrozooplankton, prey
organisms in level III as micronekton, and prey organisms (seabirds
themselves) in level IV as macronekton (after Sverdrup et al. 1942).
[Illustration: Fig. 2. Schematic food web of the parakeet auklet in the
eastern Bering Sea (based on Bédard 1969_a_ and Dunbar 1946). Arrow
sizes indicate relative importance of prey and Roman numerals refer to
prey sizes (see text).]
Table 11. _Use of food resources by seabirds in the Bering Sea
coastal domain._ Information is from Tables 1-10. (Trophic level
I = plants, II = secondary carnivore, III = tertiary carnivore,
IV = upper level carnivore [on birds only in this table], Sc =
scavenger on carrion, offal, or detritus [II-IV]; x = major food
in diet, o = minor food, * = incidental food, ? = probable food.)
-----------------------------------------------------------------------------
Habitat, bird trophic levels (I-IV. Sc), and food categories
-----------------------------------------------------------------------------
Oceanic and offshore neritic Inshore neritic Inshore neritic
-----------------------------------------------------------------------------
Key:
II II III
A. Crustacean J. Crustacean, midwater R. Fish, midwater
B. Polychaete K. Crustacean, benthic S. Fish, benthic
C. Coelenterate L. Coelenterate T. Cephalopod
D. Fish/squid eggs & larvae M. Echinoderm IV. U. Birds
III N. Mollusc Sc. V. Carrion/offal/
E. Fish Q. Fish/squid eggs & larvae detritus
F. Cephalopod
IV G. Birds
Sc H. Carrion/offal/detritus
I I. Plant
Seabirds | A B C D| E F G H I| J K L |M N Q |R S T |U V |
----------------------------------------------------------------------------|
_Gavia adamsii_ * o x
_G. arctica_ o x
_Podiceps grisegena_ o o x
_Diomedea nigripes_ x o o o x x x
_Fulmarus glacialis_ x o x o x x x
_Puffinus griseus_ x x x o o
_P. tenuirostris_ x o x o o
_Oceanodroma furcata_ x o o x x x x
_Phalacrocorax auritus_ o x o
_P. pelagicus_ x x
_P. urile_ x x
_Branta bernicla_ x
_Philacte canagica_ x
_Clangula hyemalis_ o x o o
_Histrionicus histrionicus_ o x o
_Polysticta stelleri_ o x o o
_Samateria mollissima_ x o x
_S. spectabilis_ o o o x
_S. fischeri_ x x
_Melanitta deglandi_ x o
_M. nigra_ o o x
_Haliaeetus leucocephalus_ x x x
_Falco peregrinus_ x
_Phalaropus fulicarius_ x x o x x
_Lobipes lobatus_ x x o x x
_Stercorarius_ spp. o x x ? x x x x
_Larus hyperboreus_ o o o o o o o o o o x x x
_L. glaucescens_ o o o o o o o o o o x x x
_L. argentatus_ o o o o o o o o o o x o x
_L. canus_ x o o o x x
_Rissa tridactyla_ x x x o
_Xema sabini_ x x o o
_Sterna paradisaea_ x x o o
_Uria aalge_ x o x x o o x o
_U. lomvia_ x o x x o x o x
_Lunda cirrhata_ ? x x
_Fratercula corniculata_ * x x
_Cepphus columba_ o x o
_Synthliboramphus antiquus_ x x o
_Brachyramphus brevirostris_ x o
_Cyclorrhynchus psittaculus_ x o * *
_Aethia cristatella_ x x
_A. pusilla_ x o
-----------------------------------------------------------------------------
Table 12. _Use of food resources by seabirds in the oceanic
and offshore neritic habitats, Bering Sea domain._ Information
is from Tables 1-10. (Trophic level I = plants, II = secondary
carnivore, III = tertiary carnivore, IV = upper level carnivore;
Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major
food in diet, o = minor food, * = incidental food, ? = probable
food.)
Bird trophic levels and food categories
II III IV Sc
---------------------------------------------------------------------------
Fish/Squid eggs & Larvae | Fish
Coelenterate | | | Cephalopod
Polychaete | | | | | Birds
Crustacean | | | | | | | Carrion/offal
Seabirds | | | | | | | | | /detritus
---------------------------------------------------------------------------
_Diomedea nigripes_ x o o o x x x
_D. immutabilis_ x
_Fulmarus glacialis_ x o x o x x x
_Puffinus griseus_ x x x
_P. tenuirostris_ x o x
_Pterodroma inexpectata_ x x
_Oceanodroma furcata_ x o o x x x x
_Phalaropus fulicarius_ x x o
_Lobipes lobatus_ x x o
_Stercorarius_ spp. o x x ? x
_Larus hyperboreus_ x o o o x x ? x
_L. glaucescens_ x o o o x x ? x
_Rissa tridactyla_ x x x o
_R. brevirostris_ x x x o
_Xema sabini_ x x o o
_Sterna paradisaea_ x x o o
_Uria aalge_ x o x x
_U. lomvia_ x o x x
_Lunda cirrhata_ ? x x
_Fratercula corniculata_ * x x
_Synthliboramphus antiquus_ x x
_Cyclorrhynchus psittaculus_ x o * *
_Aethia cristatella_ x x
_A. pusilla_ x o
_A. pygmaea_ x
-----------------------------------------------------------------------------
Information contained in Tables 11-15 can be summarized to show
characteristics of seabird trophic relations. One such characteristic
is the range of diet breadth or diet complexity (Table 16). Few species
(about 6%) feed on only one type of prey and might, therefore, be
referred to as "specialists." Included are eared grebe _(Podiceps
caspicus)_, Laysan albatross, brown pelican, emperor goose _(Philacte
canagica)_, black brant _(Bernicia bernicla)_, peregrine falcon, and
whiskered auklet _(Aethia pygmaea)_. Consideration of these species
as specialists may require revision when more data become available.
Except for the albatross and auklet, these species are members of
the inshore neritic cohort. Food specialization does not seem to be
characteristic of oceanic birds in particular or of most seabirds in
general.
Table 13. _Use of food resources by seabirds in the Alaska Stream
domain._ Information is from Tables 1-10. (Trophic level I =
plants, II = secondary carnivore, III = tertiary carnivore, IV
= upper level carnivore, Sc = scavenger on carrion, offal, or
detritus [II-IV]; x = major food in diet, o = minor food, * =
incidental food, ? = probable food.)
----------------------------------------------------------------------------
Habitat, bird trophic levels (I-IV. Sc), and food categories
Key:
Oceanic and offshore neritic
II
A: Crustacean
B: Polychaete
C: Coelenterate
D: Fish/squid eggs & larvae
III
E: Fish
F: Cephalopod
IV
G: Birds
Sc
H: Carrion/offal/detritus
I
I: Plant
Inshore neritic
II
J: Crustacean, midwater
K: Crustacean, benthic
L: Coelenterate
M: Echinoderm IV:
N: Mollusc Sc:
Q: Fish/squid eggs & larvae
III
R: Fish, midwater
S: Fish, benthic
T: Cephalopod
IV
U: Birds
Sc
V: Carrion/offal/detritus
Seabirds A B C D E F G H I J K L M N Q R S T U V
----------------------------------------------------------------------------
_Gavia immer_ * x x
_G. adamsii_ * o x
_G. stellata_ * o x
_Podiceps grisegena_ o x o
_Diomedea nigripes_ x o o o x x x
_Fulmarus glacialis_ x o x o x x x
_Puffinus griseus_ x x x o
_P. tenuirostris_ x o x o
_Pterodroma inexpectata_ x x
_Oceanodroma furcata_ x o o x x x x
_Phalacrocorax auritus_ o x o
_P. pelagicus_ x x
_P. urile_ x x
_Philacte canagica_ x
_Clangula hyemalis_ o x o
_Histrionicus histrionicus_ x x o
_Polysticta stelleri_ o x o o
_Somateria mollissima_ x o x
_S. spectabilis_ o o x
_S. fischeri_ x x
_Melanitta deglandi_ x o
_M. perspicillata_ o x o
_M. nigra_ o o x
_Mergus serrator_ o x x
_Haliaeetus leucocephalus_ x x x
_Falco peregrinus_ x
_Phalaropus fulicarius_ x x o x x
_Lobipes lobatus_ x x o x x
_Stercorarius_ spp. o x x ? x x x x
_Larus hyperboreus_ o o o o o o o o o o x x x
_L. glaucescens_ o o o o o o o o o o x x x
_L. argentatus_ o o o o o o o o o o x o x
_L. canus_ x o o o x x
_Rissa tridactyla_ x x x o
_R. brevirostris_ x x x o
_Sterna paradisaea_ x x o o x o
_S. aleutica_ x o
_Uria aalge_ x o x x o o x o
_U. lomvia_ x o x x o x o x
_Lunda cirrhata_ ? x x
_Fratercula corniculata_ * x x
_Cepphus columba_ o x o
_Brachyramphus marmoratus_ x o x
_B. brevirostris_ x o
_Synthliboramphus antiquus_ x x x
_Cyclorrhynchus psittaculus_ x o * *
_Aethia cristatella_ x x
_A. pusilla_ x o
_A. pygmaea_ x
Table 14. _Use of food resources by seabirds in the oceanic
habitat, central subarctic domain._ Information is from Tables
1-10. (Trophic level I = plants, II = secondary carnivore, III =
tertiary carnivore, IV = upper level carnivore, Sc = scavenger on
carrion, offal, or detritus [II-IV]; x = major food in diet, o =
minor food, * = incidental food, ? = probable food.)
--------------------------------------------------------------------
Bird trophic levels and food categories
---------------------------------------
II III IV Sc
-------------- ------- --- ------------
|Crustacean
| |Polychaete
| | |Coelenterate
| | | |Fish/squid eggs & larvae
| | | | |Fish
| | | | | |Cephalopod
| | | | | | |Birds
Seabirds | | | | | | | |Carrion/offal/
| | | | | | | | detritus
--------------------------------------------------------------------
_Diomedea nigripes_ x o o o x x x
_D. immutabilis_ x
_Fulmarus glacialis_ x o x o x x x
_Puffinus carneipes_ o x x
_P. griseus_ x x x
_P. tenuirostris_ x o x
_Pterodroma inexpectata_ x x
_Oceanodroma furcata_ x o o o x x x
_O. leucorhoa_ x o o o x x x
_Phalaropus fulicarius_ x x o
_Lobipes lobatus_ x x o
_Stercorarius_ spp. o x x ? x
_Larus hyperboreus_ x o o o x x ? x
_L. glaucescens_ x o o o x x ? x
_L. argentatus_ x o o o x x x
_Rissa tridactyla_ x x x o
_Xema sabini_ x x o o
_Sterna paradisaea_ x x o o
_Uria aalge_ x * x x
_U. lomvia_ x * x x
_Lunda cirrhata_ o x x
_Fratercula corniculata_ * x x
_Cerorhinca monocerata_ x x
_Synthliboramphus antiquus_ x x
_Cyclorrhynchus psittaculus_ x o * *
_Ptychoramphus aleuticus_ x o
--------------------------------------------------------------------
Most species (roughly 53% in any community) include two or three
prey categories in their diets--usually midwater schooling fish,
squid, and crustaceans. These birds include the most numerous in the
communities--the shearwaters and some alcids--which feed largely on
three prey types, and also include some of the less abundant birds, the
marine ducks, which feed mostly on two prey categories.
The remaining seabirds are more general in their feeding. Many have
large populations, but are not as abundant as shearwaters or most
alcids. The true "generalists" are the species that feed on as many
as eight or more types of prey, and relatively few (12%) such species
exist in each avian community. These birds, the scavengers, include
black-footed albatross, fulmar, storm-petrels, and large gulls. The
petrels are the scavengers of the oceanic habitat and the gulls are
their counterparts in the neritic habitat (but see Sanger 1973).
Another comparison is shown in Table 17, where the species in each
community are categorized according to the number feeding at each
trophic level. If a species feeds at more than one level, it is
tallied once in each level. Most seabirds (66-77%) feed at the second
and third levels as secondary and tertiary carnivores. Few feed as
terminal carnivores, and relatively few are scavengers. Actually, most
scavenging occurs at levels II and III, so about 90% of the seabirds
in each community feed at levels II and III. Communities including
an inshore neritic feeding element are the only ones that include
herbivores, and even then, few of these species exist in significant
numbers in the marine environment (discounting estuaries and sheltered
bays).
Table 15. _Use of food resources by seabirds in the North
American coastal domain._ Information is from Tables 1-10.
(Trophic level I = plants, II = secondary carnivore,
III = tertiary carnivore, IV = upper level carnivore,
Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major
food in diet, o = minor food, * = incidental food, ? = probable
food.)
-----------------------------------------------------------------
Habitat, bird trophic levels (I-IV, Sc), and food categories
-----------------------------------------------------------------
Key:
Oceanic and offshore neritic
II
A: Crustacean
B: Polychaete
C: Coelenterate
D: Fish-squid eggs & larvae
III
E: Fish
F: Cephalopod
IV
G: Birds
Sc
H: Carrion/offal/detritus
I
I: Plant
Inshore neritic
II
J: Crustacean, midwater
K: Crustacean, benthic
L: Coelenterate
M: Echinoderm
N: Mollusc
O: Fish/squid eggs & larvae
III
P: Fish, midwater
Q: Fish, benthic
R: Cephalopod
IV
S: Birds
Sc
T: Carrion/offal/detritus
Seabirds A B C D E F G H I J K L M N O P Q R S T
---------------------------+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
_Gavia immer_ * x x
_G. adamsii_ * o x
_G. arctica_ o x
_G. stellata_ o x
_Podiceps grisegena_ o x o
_P. nigricollis_ x o
_P. auritus_ x x o
_Aechmophorus occidentalis_ o x x
_Diomedea nigripes_ x o o o x x x
_Fulmarus glacialis_ x o x o x x x
_Puffinus creatopus_ o x x
_P. carneipes_ o x x
_P. bulleri_ x x x
_P. griseus_ x x x o o o
_P. tenuirostris_ x o x o o o
_Oceanodroma furcata_ x o o o x x x
_Pelecanus occidentalis_ x
_Phalacrocorax auritus_ o x o
_P. penicillatus_ o x
_P. pelagicus_ x x
_Branta bernicla_ x
_Clangula hyemalis_ o x o o
_Histrionicus histrionicus_ o x o
_Melanitta deglandi_ x o o
_M. perspicillata_ o x o
_M. nigra_ * o o x
_Mergus serrator_ * x x
_Haliaeetus leucocephalus_ x x x
_Falco peregrinus_ x
_Phalaropus fulicarius_ x x o x x o
_Lobipes lobatus_ x x o x x o
_Stercorarius_ spp. o x x ? x x x o x
_Larus hyperboreus_ o o o o ? o o o o o o x x x
_L. glaucescens_ o o o o ? o o o o o x x x x
_L. occidentalis_ x x x x o x o o o * * x x o x
_L. argentatus_ o o o o o x o o o o o x o x
_L. heermanni_ x x x
_L. canus_ x o o o x x x
_L. philadelphia_ x x o
_Rissa tridactyla_ x x x o
_Xema sabini_ x x o o
_Sterna paradisaea_ x x o o
_S. hirundo_ o x
_Uria aalge_ x o x x o x o
_U. lomvia_ x o x x o x o x
_Lunda cirrhata_ ? x x
_Fratercula corniculata_ * x x
_Cerorhinca monocerata_ x x
_Cepphus columba_ o x o
_Brachyramphus marmoratus_ x o x
_B. brevirostris_ x o
_Synthliboramphus antiquus_ x x o
_Ptychoramphus aleuticus_ x x
---------------------------+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Table 16. _Number of seabirds of different oceanographic regions
having different numbers of categories of food in their diets._
----------------------------------------------------------------------------
Number of categories in the diets[36]
------------------------------------
Oceanographic region (domain) 1 2 3 4 5-7 7 8+
----------------------------------------------------------------------------
Bering Sea coastal 3 11 9 6 5 4 5
Bering Sea 2 6 5 7 0 5 0
Alaskan Stream 3 14 14 5 4 4 5
Central Subarctic 1 6 8 4 0 7 0
North American Coastal 3 14 17 6 3 4 6
Total 12 51 53 28 12 24 16
Percent total species (196) 6 26 27 14 6 12 8
----------------------------------------------------------------------------
It is readily apparent from the foregoing comparisons that much overlap
exists in the prey eaten by seabirds within each community. The
question whether real competition ever exists is academic. Competition
perhaps exists only rarely because seabirds partition resources through
use of different feeding methods, selection of different-sized prey,
and habitat zonation. Table 18 lists feeding methods (after Ashmole
1971 and Ainley 1977) and the body size and bill length of each species
considered in this review. Bill length is usually related directly to
body size (Ashmole 1968; Bédard 1969_b_), but note, for instance, that
the longer species of the two kittiwakes has the shorter bill. Body
weight would be a better measure of relative size than body size, but
few reliable weight data are available for seabirds.
The use of different feeding methods by species in each community
grossly assigns birds to feeding at different depths. Thus, whereas
shearwaters, puffins, and small gulls (_Xema_ sp., _Rissa_ spp.)
overlap almost entirely in prey categories and even prey species, the
gulls can capture these organisms only at the surface; the shearwaters
capture them at shallow depths; and the puffins capture them at much
deeper depths. Direct field observations of this phenomenon are few but
Gould (1971) and Sealy (1973_a_) compared the diets of birds feeding
in mixed-species flocks. An example of how even finer divergence in
feeding methods helps to partition food resources has been provided by
Spring (1971) in his comparison of the two murres. Both species feed
by diving to great depths, but the thick-billed murre is able to hover
over the bottom and thereby is better able to capture benthic organisms.
Table 17. _Number of species feeding at different trophic levels
within seabird communities and habitats of the northeastern North
Pacific Ocean and Bering Sea._ A single species can be represented
in more than one level. (Trophic level I = vegetarian,
II = secondary carnivore, III = tertiary carnivore, IV = upper
level carnivore, Sc = scavenger [II-IV].)
------------------------------------------------------------------------------
Oceanic/offshore neritic Inshore neritic
------------------------- ----------------------------
Domain II III IV Sc I II III IV Sc
------------------------------------------------------------------------------
Bering Sea Coastal 11 17 1? 10 6 23 18 6 6
Bering Sea 22 21 3? 11 -- -- -- -- --
Alaska Stream 21 19 1? 12 5 28 21 6 6
Central Subarctic 23 22 3? 12 -- -- -- -- --
North American Coastal 25 24 3? 11 3 28 35 7 10
Total 102 103 11? 56 14 79 74 19 22
Proportion 0.38 0.39 0.02[37]0.21 0.07 0.38 0.28 0.09 0.10
------------------------------------------------------------------------------
Table 18. _Size relationships and feeding methods of major
species in the eastern North Pacific and Bering Sea._ (D = dive,
SS = surface seize, PP = pursuit plunge, Di = dip, P = plunge,
T = tip, x = eats seabirds, A = piracy, SP = shallow plunge.)
--------------------------------------------------------------------
Body length[a] Bill length[b] Feeding[c]
Species (cm) (mm) method
--------------------------------------------------------------------
_Gavia adamsii_ 63.5 90-91 D
_G. immer_ 61.0 80-82 D
_G. arctica_ 45.7 51-52 D
_G. stellata_ 43.5 51-52 D
_Podiceps grisegena_ 33.0 48-50 D
_P. nigricollis_ 22.9 24-26 D
_P. auritus_ 24.1 23-24 D
_Aechmophorus occidentalis_ 45.7 65-76 D
_Diomedea nigripes_ 71.1 141-144 SS
_D. immutabilis_ 71.1 102-112 SS
_Fulmarus glacialis_ 45.7 36-37 SS
_Puffinus carneipes_ 45.7 41-46 PP
_P. creatopus_ 45.7 41-46 PP
_P. bulleri_ 38.1 38-45 PP
_P. griseus_ 40.3 41-42 PP
_P. tenuirostris_ 38.1 31-32 PP
_Oceanodroma furcata_ 19.0 15 Di,SS
_O. leucorhoa_ 19.0 16 Di,SS
_Pterodroma inexpectata_ 29.2 26-27 SS
_Phalacrocorax auritus_ 68.6 55-57 D
_P. penicillatus_ 73.7 66-71 D
_P. urile_ 71.1 54-55 D
_P. pelagicus_ 55.9 47-50 D
_Pelecanus occidentalis_ 104.0 294-319 P
_Branta_ spp. _(bernicla)_ 43.5 33-36 T
_Philacte canagica_ 45.7 37-42 T
_Anas_ spp. 40.0 32-35 T
_Clangula hyemalis_ 38.1 25-27 D
_Histrionicus histrionicus_ 30.5 25-28 D
_Polysticta stelleri_ 30.5 37-43 D
_Somateria mollisima_ 43.5 45-55 D
_S. spectabilis_ 40.3 31-33 D
_S. fischeri_ 38.1 22-26 D
_Melanitta deglandi_ 35.6 41-44 D
_M. perspicillata_ 40.3 ca. 40 D
_M. nigra_ 35.6 42-47 D
_Mergus serrator_ 40.3 45-54 D
_Haliaeetus leucocephalus_ 80.0 52-54 X
_Falco peregrinus_ 37.5 21-25 X
_Phalaropus fulicarius_ 16.5 22 SS
_Lobipes lobatus_ 15.2 22 SS
_Stercorarius pomarinus_ 43.5 40 SS,A
_S. parasiticus_ 40.3 32 SS,A
_S. longicaudus_ 38.1 29 SS,A
_Larus hyperboreus_ 61.0 55-60 SS
_L. glaucescens_ 55.9 54-58 SS
_L. occidentalis_ 53.0 54-57 SS,Di
_L. argentatus_ 50.8 48-54 SS,Di
_L. californicus_ 43.5 45-50 SS,Di
_L. heermanni_ 38.1 42-46 SS,Di
_L. canus_ 35.6 34-36 SS,Di
_L. philadelphia_ 27.9 30-31 Di
_Rissa tridactyla_ 34.2 39-40 Di
_R. brevirostris_ 38.1 29-30 Di
_Xema sabini_ 27.9 26-27 Di
_Sterna paradisaea_ 38.1 31-33 Di,SP
_S. hirundo/forsteri_ 35.6 36-39 Di,SP
_S. aleutica_ 33.0 33 Di,SP
_Uria aalge_ 35.6 43-47 D
_U. lomvia_ 35.6 39-42 D
_Lunda cirrhata_ 31.8 57-60 D
_Fratercula corniculata_ 29.2 49-51 D
_Cerorhinca monocerata_ 29.2 34-35 D
_Cepphus columba_ 26.7 32-33 D
_Brachyramphus marmoratus_ 20.3 15 D
_B. brevirostris_ 19.0 10 D
_Synthliboramphus antiquus_ 20.3 13 D
_Ptychoramphus aleuticus_ 17.8 19 D
_Aethia pygmaea_ 16.5 8-9 D
_A. pusilla_ 13.3 8 D
_A. cristatella_ 17.8 11 D
_Cyclorrhynchus psittaculus_ 18.4 15 D
--------------------------------------------------------------------
The scavengers (generalists) offer a good example of how a range of
bird and bill sizes is usually represented among species having similar
diets and feeding methods. The progression of oceanic scavenger sizes
is graded rather evenly from the black-footed albatross down to the
northern fulmar, to the scaled petrel, to the storm-petrel. All these
species capture prey that occur only at or near the water surface.
Recently Sanger (1973) reported appreciable numbers of glaucous-winged
gulls _(Larus glaucescens)_ and herring gulls _(L. argentatus)_,
noted neritic scavengers, out in the oceanic realm of the petrel.
He presented limited data that suggested an overlap between the
diet of these gulls and that of black-footed albatrosses, as noted
by Miller (1940). It would not be surprising if these gulls were as
much generalists in the oceanic habitat as they are in the neritic.
Interestingly, their bill and body sizes fall between those of the
albatross and the fulmar, thus in theory enabling them to invade the
oceanic habitat without great competition. It is likely that their
invasion occurred during historical times and is related to their habit
of following fishing boats from shore out to sea (Sanger 1973). If so,
the gulls might be assuming from other species part of a previously
uncontested resource.
Another interesting group of species that shows close similarities
in diet consists of the piscivorous loons, grebes, and mergansers.
All these birds, including seven or eight species, apparently feed on
fish occurring on or near the bottom in the inshore neritic habitat.
Again, however, an even progression in size exists: yellow-billed
loon _(Gavia adamsii)_, common loon _(G. immer)_, arctic loon
_(G. arctica)_, red-throated loon _(G. stellata)_, western grebe
_(Aechmophorus occidentalis)_, red-necked grebe _(Podiceps grisegena)_,
and common merganser _(Mergus merganser)_. Most likely then, they
select different-sized fish. Another example of this phenomenon is
provided by the eight neritic gulls, which are largely scavengers and
show a remarkably even progression in bill and body size. Finally, as
shown clearly by Bédard (1969_a_, 1969_b_) and Harris (1970), alcids of
different sizes select different-sized prey, often of the same species.
A final important way in which seabirds partition available resources
is by inhabiting different zones. Zonation is especially evident during
the breeding season when species common to the same breeding site
sort themselves out according to the distances they range for food.
This phenomenon was discussed by Murphy (1936), Shuntov (1974), Sealy
(1972), Cody (1973), and Scott (1973).
Trophic Relations and Seabird Conservation
The species that appear to have specialized food habits (if further
research confirms that indeed they do) are probably very sensitive
to vagaries in food availability or are, at least, much more
sensitive than other species. Some specialists which also have very
restricted distributions would, therefore, be susceptible to localized
catastrophes occurring where specialists are concentrated around the
food resource. This is proved in the case of the scoters, which are
both specialized and rather restricted to nearshore beds of molluscs
and have fallen victim to local oil slicks (Smail et al. 1972). An
example of another potentially critical situation is that of the black
brant, which at certain times of the year concentrate their entire
population around eelgrass beds in Bristol Bay, Alaska, where much
offshore oil drilling may soon occur.
Birds adapted to feed by diving, with the exception of cormorants,
spend most of their time in the water. These species are therefore
most susceptible to oiling (Smail et al. 1972), but pursuit plungers
(the shearwaters) are also highly susceptible (Point Reyes Bird
Observatory, unpublished data). A characteristic of polar and subpolar
seabird communities is the high percentage of birds that feed by
diving and pursuit plunging. These birds are mostly absent from
tropical and subtropical communities because feeding by these methods
is not adaptive there (Ainley 1977). Hence, oil pollution has all the
potential of rendering maladaptive the principal feeding methods of
many polar seabirds.
Another way in which seabird feeding relates to conservation problems
concerns competition between birds and man for commercially valuable
fishes. A related problem is the mass mortality of seabirds due to
man's fishing gear. An acute situation is the drowning of seabirds
caught in salmon gill nets (Bartonek et al. 1974; Pacific Seabird Group
1975; Ripley 1975; King et al., this volume). Immediate action is
definitely required.
Further, competition between birds and man for the same resource has
the potential for disastrous effects on bird populations if humans
out-compete the birds and overfish the resource. A classic example,
reviewed by Idyll (1973), is the possible collapse of the Peruvian
anchovy _(Engraulis ringens)_ fishery; if overfishing and an El Niño
should coincide, the Peruvian seabird populations could collapse as
well. The California fisheries and apparently the double-crested
cormorants that nest on the Farallon Islands have both suffered
from the demise of the Pacific sardine _(Sardinops caerulea)_ in
the California current (Ainley and Lewis 1974). In regulating fish
harvests, fishery organizations should include in their calculations
the harvest by creatures other than man (Schaefer 1970), rather than
evading the issue by referring to a vague "natural mortality."
* * * * *
Finally, fishing by humans can benefit seabirds by removing fish (or
whales) that compete with birds for food (Laws 1977). A potential
example is that of northern California, where salmon and seabirds both
feed heavily on juvenile rockfishes (Fitch and Lavenberg 1971; Point
Reyes Bird Observatory, unpublished data). Harvest of salmon should
theoretically leave more rockfish available for birds to eat. This sort
of situation has not yet been fully documented and definitely warrants
further study, especially in such areas as the Bering Sea, where some
fish stocks have become depressed due to overfishing (Gulland 1970).
Recommendations for Further Research
Many people realize intuitively that seabirds are important members
of marine ecosystems. Although the supporting evidence is not now
available, it will be needed if seabirds are to be protected. Emotion
alone will not justify the protection of seabirds in an age when the
human race moves steadily toward global famine. The job at hand is, in
part, to sell seabirds, not just to the public, government officials,
executives of oil companies, or fish-packing concerns, but also to
marine biologists and oceanographers, for the scientists have the best
means to study organisms at sea. We must move away from the concept
that seabirds are merely yo-yos of various sizes, shapes, and colors
on strings of various lengths that venture forth to sea from the land,
grab a quick lunch, and then return to the safety of terra firma.
Seabirds are marine organisms and deserve at least as much research
attention as that currently given marine mammals.
The information now available on seabird diets is largely presented in
terms of the number and volume of various prey species taken. Whereas
these data provide the relative importance of prey, fishery data on
prey stocks are usually measured in terms of biomass. Thus, it is
difficult to relate seabird data to the immense wealth of information
on biological oceanography. If we are to recognize the importance of
seabirds in the nutrient and energy cycling of marine ecosystems,
rather than considering them merely as "yo-yo predators," we must
relate them to the total marine community.
The goal of marine ornithologists should be to refine and broaden
considerably in detail such studies as those by Sanger (1972), Shuntov
(1974), and Laws (1977), who attempted to assess the relations
between seabird populations and stocks of other marine organisms for
the northern North Pacific, the world oceans, and the Antarctic,
respectively. The trophic roles played by seabirds must be studied in
detail at the community level year-round before those analyses can
be properly refined. Another exemplary work is that done by Brownell
(1974), who studied trophic relations of higher vertebrates off
Uruguay, including dolphins, pinnipeds, seabirds, and some large fish.
In a review study, Sanger (1974) considered the food-chain relations
of similar vertebrates in the Bering Sea. These sorts of studies will
serve to bring the role of seabirds into perspective with other upper
trophic level feeders.
Acknowledgments
We much appreciate the opportunity to participate in the symposium at
which this paper was presented. The encouragement and help given by
J. C. Bartonek was indispensable. D. G. Ainley's participation in the
symposium was supported by the Point Reyes Bird Observatory. This is
contribution No. 124 of the Point Reyes Bird Observatory.
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FOOTNOTES:
[15] Present address: U.S. Fish and Wildlife Service, Office of
Biological Services, 1011 East Tudor Road, Anchorage, Alaska 99503.
[16] Other incidental prey were squid and atherinid fishes, both at the
Farallon Islands.
[17] Other incidental prey were squid and such fishes as atherinids,
_Zaniolepis_, _Genyonemus_ and _Peprilus_ at the Farallones, and
atherinids, _Trachurus_ and _Heterostichus_ at San Diego.
[18] Other incidental prey were polychaetes at Netarts and the Farallon
Islands.
[19] Principal sources: Bent 1925; Cleaver and Franett 1945; Cottam
1939; Cottam and Knappen 1939; Kortright 1942; Mabbot 1920; McGilvrey
1967; Munro and Clemens 1939; Roberts and Huntington 1959.
[20] Other incidental items were the fish _Cololabis_ and _Peprilus_ at
the Farallon Islands.
[21] Other incidental items were myctophid fish in northeastern Canada.
[22] Other incidental items included the lamprey _(Lampetra)_ at the
Farallon Islands.
[23] Bent (1946) listed "fish" as prey.
[24] Grinnell (1897) listed "fish" as the major dietary component.
[25] Bédard (1969_a_) also listed "fish" as an incidental item.
[26] Other incidental prey were copepods and isopods.
[27] Other incidental prey were pholids in Denmark.
[28] Other incidental prey were copepods and cephalopods in North
Atlantic areas.
[29] Other incidental prey were isopods in western North America and
fish eggs near Vancouver Island.
[30] Other incidental prey were fish eggs in Denmark.
[31] Offal from wounded whales and seals, and bits of food, primarily
crustaceans and fish, from feeding whales are important scavenger foods
(Bent 1922).
[32] Other incidental prey were isopods in the North Pacific.
[33] Other incidental prey were the fish _Merluccius_ at the Farallon
Islands.
[34] Other incidental prey were isopods near the Pribilofs and in the
Chukchi Sea, and amphipods in the latter area; Bent (1921) considered
"crustaceans" to be major prey.
[35] Study conducted during period of breeding failure.
[36] These are the "food categories" of Tables 11-15. Items included in
diets are not included here.
[37] Proportion based on the arbitrary assumption that half (5) of the
11 species in question catch and eat birds at sea.
[38] Information on body sizes (length) is from Robbins et al. (1966).
[39] Information on bill lengths is from Palmer (1962), Dement'ev et
al. (1968), and Friedmann (1950).
[40] Feeding methods are from Ashmole (1971) as adapted by Ainley
(unpubl. manuscr.).
Population Dynamics in Northern Marine Birds
by
William H. Drury
College of the Atlantic
Bar Harbor, Maine 04609
Abstract
It seems only reasonable to assume that
populations of marine birds fluctuate even
when not disturbed by man; such fluctuations
would result both from the secondary effects
of species adaptive tactics and from changes
in the marine environment. I briefly review
some human activities and some other natural
processes that have resulted in changes in
numbers and distribution of seabirds and
present a short discussion of theoretical
models which emphasizes that conclusions drawn
or predictions made from models of the dynamics
of populations depend upon the assumptions
about stability that were used in preparing
the models. I then review those special
characteristics of seabirds which are directly
relevant to planning programs intended to
protect seabirds or encourage their increase
and identify several goals for improving our
understanding of the population dynamics and
biology of marine birds. My general conclusion
is that enough is already known to undertake
effective conservation programs, and that time
is pressing.
Seabirds have been categorized as renewable resources in only a few
places, although their symbolic value has been recognized for centuries
(for example, the medieval poem "The Seafarer" and the designs on Saint
Cuthbert's tunic). With the exception of the Russians (Belopol'skii
1961; Uspenski 1956), the Australians (Serventy 1967), and the
Icelanders, industrialized peoples have not considered seabirds to be
salable and therefore worth managing. Yet during many centuries the
seabirds of the northern seas were a major food for coastal and island
villages (Bent 1919, 1921, 1922; Fisher and Lockley 1954).
Some biological principles that affect the dynamics of seabird
populations are identified in this paper. I believe these principles
must form the basis of plans to maintain and increase seabird numbers.
I describe some observations of population changes, review briefly
the conflicting theoretical frameworks for population dynamics, and
identify some of the biological characteristics of marine birds that
affect the way in which population changes occur. The terms "seabirds"
and "marine birds" are used interchangeably for those bird species
which depend upon salt water for some part of their annual cycle (c.f.,
the Pacific Seabird Group).
Population Fluctuations
Broadly stated, the populations of northern seabirds have shown marked
short-and long-term fluctuations. Most authors have assumed that all
such fluctuations reflect human disturbance of the natural system,
because of the obvious effects of human predation during the last 200
years.
_Human Impact_
In the centuries before people traveled extensively between islands,
seabirds were taken in ways that we judge must have allowed the
survival of the colonies (e.g., those at the Faroes or Saint Kilda,
those in Iceland and Greenland, or those in the Aleutian Islands and
the Bering Strait). We presume either that the populations of island
peoples were regulated by shortage of resources other than seabirds
or that those who overcropped and eliminated the seabirds suffered the
consequences.
Negative Effects
When a sea-going, commodity-oriented way of life evolved, seabirds were
killed in huge numbers for such uses as the plumage trade, fish bait,
or rendering into oil (Tuck 1960; Fisher and Lockley 1954). Even the
elimination of several colonies--e.g., Funk Island, Newfoundland (Tuck
1960); Seal Island, Eastern Egg Rock, Maine (Norton 1921); Muskeget,
Massachusetts (Forbush 1929)--may have had little effect on the rate
of cropping because those who killed off one source could probably
seek out another. As the colonial seabirds became scarce they became
more valuable, which stimulated more intensive pursuit of the remnants
(Dutcher 1901, 1904).
In some places where seabird colonies did not supply a croppable
economic resource, the islands were used for alternative crops with
at least temporary commodity value (e.g., foxes were introduced in
the Aleutian Islands; Bent 1919). Large herbivores were introduced to
supply meat for island residents (e.g., Saint Matthew Island; Klein
1959), as well as pigs, cattle, sheep, goats, and rabbits on islands
in the North Atlantic and southern oceans (many authors). Increases in
many seabird populations over the last 75 years have been generally
associated with relief from predation by humans such as the fowlers,
eggers, and plume hunters of the 19th century. Such relief may have
been partly responsible for the increase of North Atlantic gannets,
_Sula bassana_, and common murres or guillemots, _Uria aalge_ (Fisher
and Vevers 1943, 1944; Cramp et al. 1974). On a smaller scale, several
population increases along the coast of New England have been recorded
following the enactment of protective legislation (Dutcher 1901, 1904;
Norton 1921, 1924; Palmer 1949; Drury 1973).
Coulson (1974) argued that in addition to relief from predation, the
explosion of the population of kittiwakes _(Rissa tridactyla)_ in this
century resulted from access to previously un-occupiable breeding
sites. Nesting cliffs and buildings suitable for kittiwake nesting are
abundant and now protected from egging or fowling.
Positive Effects
There can be little doubt that human activities have also had marked
positive effects in some cases. For example, Fisher (1952) suggested
that the North Atlantic fulmar _(Fulmarus glacialis)_ was provided
food first by whaling, then by commercial fishing, and that this food
allowed the species to increase steadily over the last 3 centuries.
The worldwide increase of gulls (_Larus argentatus_, _L. fuscus_, _L.
dominicanus_, _L. ridibundus_, _L. novae-hollandii_) has been credited
to availability of food from wasteful human garbage disposal (Murray
and Carrick 1964; Fordham 1968, 1970; Harris 1964; Harris and Plumb
1965; Kadlec and Drury 1968; Brown 1967; Mills 1973; Vermeer 1963).
It is hard to dismiss the evidence pointing to the impact of human
activities on seabird populations during the last 3 centuries. Yet
it would be misleading to assume that without man's interference
seabird populations would have remained stable. Success in designing
programs of protection and population enhancement must allow for the
realities--that seabird populations fluctuate inherently, and that
secular changes occur regularly in their environment.
_Impact of Natural Events_
Some population changes appear to result from sudden impacts; other
changes are gradual.
Sudden Disasters
Gromme (1927) reported windrows of dead murres in the Unimak Pass and
Alaska Peninsula; die-offs of murres in winter storms in the Atlantic
and Arctic Oceans were reported by Tuck (1960) and Dement'ev et al.
(1968).
Recently some mass mortalities have been associated with specific
causes. Bailey and Davenport (1972) reported that starvation caused
the die-off of common murres in the southern Bering Sea--Bristol Bay
area. Foul weather, which apparently inhibited feeding between 19
and 23 April 1970, culminated in an intense storm. Similarly in late
winter 1969 bad weather in the Irish Sea, combined with strains of
molt and perhaps contamination with industrial chemicals, seems to
have contributed to mass mortality of the same species (called common
guillemot in Britain; Holdgate 1971). The seabird victims of this
event had metabolized their body fat and as a result, polychlorinated
biphenyls (PCB) and other industrial chemicals passed into livers,
kidneys, and brains. Again, a storm at the end of a period of stress
seems to have been more than the birds could tolerate.
A further example of a die-off of waterfowl apparently brought on by
starvation was given by Barry (1968), who estimated that about 100,000
king eiders _(Somateria spectabilis)_ died when they arrived before the
ice broke up in the Beaufort Sea in spring 1964.
Diseases have produced massive die-offs in marine birds. Fowl
cholera caused high mortality in nesting common eiders _(Somateria
mollissima)_ in the Gulf of St. Lawrence in Quebec (Reed and Cousineau
1967) and in Penobscot Bay, Maine, in the early 1960's (H. Mendall,
personal communication). Poisoning from a "red tide" (a bloom of the
dinoflagellate _Gonyaulax tamerensis_) caused a die-off of black ducks
_(Anas rubripes)_ and herring gulls on the coast of New England in
1972. Similarly a die-off of shags _(Phalacrocorax aristotelis)_ on the
east coast of England was caused by a "red tide" (Coulson et al. 1968).
During a period of 1 week 90% of the shag nests on the Farne Islands in
Northumberland were deserted and about 80% of the breeding population
died.
Gradual Declines
When the new volcanic island of Bogoslov emerged in the western
Aleutians, Preble and McAtee (1923) reported that it was colonized by
large numbers of pigeon guillemots _(Cepphus columba)_, but in the
following decades the guillemots have steadily decreased (G. J. Divoky,
personal communication). As a further example, the nesting population
of Atlantic puffins _(Fratercula arctica)_ in the Atlantic has declined
over the past several years, especially those nesting on the Outer
Hebrides (Flegg 1972; Harris 1976).
It is difficult to find seabird species whose nesting grounds have
not been affected by humans but whose numbers have been censused. The
best illustrations of secular changes in relatively constant habitats
are probably those available in the British Trust for Ornithology's
breeding censuses of songbirds. Songbirds are short-lived and their
populations change on relatively short time scales. The northwestern
European landscape has remained relatively constant for the last 75
years, yet there are observable decade-long trends--for example, of
willow warblers _(Phylloscopus trochilus)_ and dunnock _(Prunella
modularis)_. There are detailed data on population changes in great
tits _(Parus major)_ through the work of Kluyver (1951), Lack (1964),
and Perrins (1965).
Effects Reflecting Environmental Change
Nelson (1966) argued that the increase of gannets in the North Atlantic
during this century has been related to increasing temperatures rather
than (as usually ascribed) to increased food from fish damaged or
escaped during commercial fishing.
Ainley and Lewis (1974) described a particularly interesting example
of the effects of environmental change on seabird populations. The
events begin with the decrease of seabirds on the Farallon Islands off
California as a result of human depredations. Even after fowling was
made illegal, the populations of murres, double-crested cormorants
_(Phalacrocorax auritus)_, and especially of tufted puffins _(Lunda
cirrhata)_ and pigeon guillemots continued to decline as a result
of oil pollution. During the last 3 decades the smaller species
of seabirds nesting on the Farallons, such as rhinoceros auklets
_(Cerorhinca monocerata)_, have increased rapidly and the authors
suggest that their increase was abetted by an increase in the small
prey fish, northern anchovy _(Engraulis mordax)_. One of course
expects predators to be affected by changes in the abundance of their
prey. During this same period, larger species of seabirds such as
double-crested cormorants and tufted puffins have failed to recover
their numbers, and the authors speculate that this failure is related
to a decrease of the larger prey fish, Pacific sardine _(Sardinops
caerulea)_.
A widely publicized impact of environmental fluctuation upon
seabird populations is that of the northeast wind, El Niño, off the
Peruvian coast. This wind pushes the upwelling Humboldt Current
water offshore and causes mass mortality in the Peruvian anchovies
_(Engraulis ringens)_ and, as a consequence, a die-off among the
millions of seabirds such as Peruvian guanay cormorants _(Phalacrocorax
bougainvillii)_ and Peruvian boobies or piquero _(Sula variegata)_
which feed upon them (Murphy 1936).
Theoretical Considerations
Can useful generalizations be drawn from these observations on
population changes? Can a model be constructed of the forces which
drive population changes or of population-habitat interactions which
keep populations from extinction? Some conflicting theories and
assumptions of population dynamics are examined and discussed below.
_The Assumption of Population Stability and of Closely Attuned
Density-dependent Mortality_
During the 5 decades before 1970, it was widely accepted that most
animal populations were generally stable and saturated before the
arrival of the white man. Although a few field biologists vigorously
dissented, "establishment" ecologists regarded fluctuations as a
departure from the norm, and as such, a hazard to the population.
Many theorists of both evolution and ecology argued that adaptations
were required to damp fluctuations or the fluctuations would become
"random walks" and the population would rapidly become extinct. As a
consequence, relatively all theoretical models included stability as a
central assumption.
• The basic element of this theoretical complex has been the
Lotka-Volterra formula for a logistic curve of population growth and
stabilization. According to this formula it has been reasoned that
by establishing the inherent rate of increase of a population (i.e.,
its average natality relative to mortality, or _r_) and by measuring
the carrying capacity of the environment (which is the density of
the population at saturation, or _K_), one can predict the maximally
productive population size, and maximum rate of production of new
individuals (or maximum sustained yield). These assumptions have
supplied the theoretical framework for virtually all game management
and many fisheries practices.
Once stability was assumed, a mechanism for maintaining stability was
necessary. This mechanism was found in an interaction between the
population and the environment, called density-dependent mortality
(Nicholson 1933). The impact of this feedback has been assumed to cause
the point of inflection of the "sigmoid curve" and to regulate the
density "at equilibrium."
Populations growing in relatively isolated or closed systems have been
observed to follow a sigmoid curve toward a steady state. We have data
on the growth of several Massachusetts gull colonies which show this
type of short-period rapid increase followed by a long sequence of
shallow oscillations (Drury and Nisbet 1972). But usually observations
have been terminated at about the time the population passed through
the point of inflection.
• Lack (1954) accepted the principles formulated by Lotka-Volterra
and hence viewed Nicholson's (1933) density-dependence as logically
necessary. Lack (1948, 1954) argued that reproductive effort (clutch
size or litter size times the number of broods) must be as large as
the parents can successfully raise to independence because these
biological characteristics are directly subject to natural selection.
He argued that because reproductive potential is excessive (Darwin
1859), mortality must be density-dependent if a population is to avoid
fluctuations. The only adequately density-dependent regulating process
he accepted was the population's response to its food supply (Lack
1954). In fact, for many years Lack rejected Kluyver and Tinbergen's
(1953) hypothesis that territory could act as a control on population
size in birds because, he argued, territories were compressible and
therefore allowed wide fluctuations. To his credit, however, Lack
eventually acknowledged this mistake.
The first defect in the concept of "carrying capacity" is the idea that
populations have "mechanisms" or "institutions" (Wynne-Edwards 1959)
by which the population is kept stable at the carrying capacity in a
stable habitat.
The second defect in the concept of carrying capacity is that it
presupposes a stable environment. During the early decades of the 20th
century most climatologists believed that a departure from the norms
of a regional climate set processes in motion which would return the
climate to normal. During the last decades, however, climatologists and
oceanographers have shown clearly that environments are continuously in
flux.
_An Attack on Density-dependent Mortality_
Some theorists rejected the concept of carrying capacity as soon as it
was formulated. Andrewartha and Birch (1954) predicted fluctuations
would be undamped by inherent population mechanisms but rather would
be controlled by external forces indifferent to the density. Their
supporting data were drawn from field studies of insects in arid
climates. Some of their ideas are directly relevant to seabirds; for
example, their assertion that in many cases limits to carrying capacity
of the habitat are not set in a way responsive to the density of the
population. The number of occupiable ledges on a seabird cliff are
fixed and when they are full no more birds can breed there regardless
of the amount of food available. For another example, some biological
processes act in a way that reinforces fluctuations. Predation can
act in this way in the relatively closed system of a seabird colony;
i.e., the smaller the prey population the larger the percentage taken
by the predators. The importance of predation as a selecting factor is
shown by the adaptations marine birds and waterfowl make to avoid it.
The fact that large colonies of seafowl are usually concentrated on
isolated, predator-free islands is one obvious case (Lack 1966).
Although their ideas are useful in understanding changes in many
species, primarily insect populations, the generality of Andrewartha
and Birch's (1954) hypothesis is weakened because it conflicts with
detailed studies of seabirds which show that in many cases local food
resources do limit breeding success. Ashmole (1963) showed this for
tropical terns, and Hunt (1972) for some colonies of herring gulls
on the New England coast. Nettleship (1972), studying the effects of
herring gulls on Atlantic puffins, showed that the effect of harassment
and stealing food from the parents was to reduce the amount of food
brought to the young and thus reproductive success. In those parts
of the colony where gulls were numerous or where the puffins were at
a disadvantage in escaping from gulls (i.e., flat rather than steep
slopes) the reproductive success of puffins was significantly lower
than in areas away from the gulls.
The literal application of Andrewartha and Birch's general ideas also
conflicts with observations on subtle adaptations some waterfowl have
made to counter predation.
Barry (1967) described the density-avoiding adaptations of
arctic-nesting geese to evade predation--specifically by foxes. Black
brant _(Branta nigricans)_ nest on low coastal or delta islands seeking
to escape by remoteness. Snow geese _(Chen caerulescens)_ are colonial
on large, flat areas, seeking protection in numbers. White-fronted
geese _(Anser albifrons)_ are solitary nesters on inland swamps,
seeking to be "over-dispersed" among scrub willow.
Common eiders, black scoters _(Melanitta nigra)_, tufted ducks _(Aythya
fuligula)_, and other ducks select gull colonies as nesting habitat.
Although there is little doubt that the ducks choose gull colonies for
nesting, there is some doubt as to the reasons. Finnish biologists
(summarized by Bergman 1957; Hildén 1965) have concluded generally that
gulls protect the duck nests from predation by hooded crows _(Corvus
corone)_.
_The Assumption that Fluctuations Are Generally Present_
Recently theorists have built models based on assumptions that
fluctuations are a general characteristic of population dynamics,
such as Gilpin's (1975) model describing multi-phased oscillations.
He took account of the fact that fluctuations (and models) become
more complex as more species and nonlinear effects are included. May
and Leonard (1975) emphasized that the effect of nonlinearities is
to make it impossible to speak even in principle of the equilibrium
point of a community. They pointed out that even though the model is
deterministic (i.e., assumes that the system will come to equilibrium)
the oscillations are so complex that they may appear to be random, and
it may be a very long time before the system returns to a position
near its starting point. "On the other hand a truly random ecological
system could always be fitted by a suitably ingenious limit cycle. This
suggests that ecological analysis which does not consider component
processes must be viewed with great suspicion" (Gilpin 1975). May
and Leonard (1975) and Gilpin are both making a familiar point--that
neither the logic nor the interactions described in a formula will
describe biological reality unless the assumptions are correct. They
are also making a different point--that an ingenious mathematician can
create a formula to describe almost any operation (whether its workings
are systematic or random), and the formula may seem to work.
Gilpin's moral is that one cannot learn very much that is helpful by
studying fluctuations as such. One must study the factors controlling
populations. This is a very old idea.
It would appear that defining carrying capacity and inherent rate of
increase will not be very instructive in managing seabird populations
other than in speculating upon what might be ideal upper limits.
It can also encourage the musty sophistry that when a population
increases beyond this abstract carrying capacity it "needs" to be
hunted to prevent overcropping resources and damage to itself through a
population decline. But we will not have the time to carry out detailed
studies of life histories seeking for critical population-habitat
interactions over several fluctuations for each species involved in a
disaster before designing programs to help seabird populations to build
up their numbers.
General Characteristics of Marine Birds and Waterfowl
Because general theory does not seem to work and because detailed
studies take too much time, I conclude that it is necessary to identify
certain general principles upon which to base applied programs. These
categories of knowledge include: (1) how vulnerable certain categories
of seabirds, waterfowl, and shorebirds are to specific types of
disasters, (2) how quickly their numbers build up after they have been
reduced, and (3) at what stages we can help them best (i.e., at the
breeding grounds, at the winter gathering grounds, or on migration). I
believe that we already know enough to design effective programs and to
begin work. To this end some characteristics of seabirds are identified
which determine the population structures and ways in which their
numbers respond to changes in the environment.
_Habitat_
Although the shallow oceans, islands, and seashores are among the
most permanent features of the earth in general, the details of
their numbers and distribution change rapidly. Sandy shores are
obviously being reworked even in the short span of a single lifetime.
Distribution of islands and the sediment load, extent, and strengths of
currents vary constantly in space and change with time.
The food that seabirds use is patchy and subject to both short-and
long-term fluctuations in numbers and shifts in geography. Suitable
breeding habitat is scattered, and in many places where oceanic
conditions provide a good food supply there are no nesting sites.
Consequently, seabirds aggregate in colonies, often dense, and the
colonies are clumped for geographical as well as biological reasons.
Lack (1966) discussed some general features of how the breeding
adaptations of seabirds are adjusted to the distances the birds
must go to find food. The species which feed close to the nest
characteristically establish isolated territories or nest in small
groups, and they accept many different kinds of nesting substrate.
Their clutch sizes are large, individuals move nesting sites readily,
and their young grow rapidly compared to the species which feed far at
sea. Species which feed far at sea aggregate in large colonies. These
species are often rigid in their requirements for suitable nesting
sites, their clutches are usually limited to one egg per season,
their young grow slowly, and there seems to be strong attachment to
traditional colony sites.
_Breeding_
Ashmole (1963) suggested that the clutch size of some oceanic birds is
small and colonies occupy only part of the available habitat because
food resources within efficient commuting distance of the breeding site
are limited. We can see this effect in the usual failure of common
terns to raise a third chick, even in the colonies that are surrounded
by favorable habitat (Nisbet 1973). Herring gulls whose colonies are
close to sources of human refuse raise more young than do those whose
colonies are at some distance (Drury 1963; Kadlec and Drury 1968; Hunt
1972).
Ashmole (1963) suggested that during the course of the breeding
season the birds exhaust the available food supply. The validity
of this suggestion is reflected in the long distances some species
(petrels, boobies, murres, dovekies) go for food to feed their young.
One would therefore expect that early nesting pairs would be more
successful, and this seems to be the case in herring gulls (Nisbet and
Drury 1972), kittiwakes (Coulson 1966), and red-billed gulls, _Larus
novaehollandiae_ (Mills 1973).
If food is in short supply and parents have to seek over a wide area
for food so that they can bring back only a little food at long time
intervals, one would expect these birds to have a small clutch and
their young to grow slowly, as is the case. One would also expect
seabird colonies situated near oceanic currents to be larger and more
successful because food is continuously renewed. Conversely, one would
expect colonies next to still waters to be smaller and less successful.
The small clutch size of seabirds means that when a population has
been reduced, it will grow slowly toward its former abundance. The
growth rates of seabird populations on the New England coast since
their release from human predation reflects this. Species such as black
guillemots with only two eggs per clutch and herring gulls with three
eggs per clutch have increased more slowly than have the populations of
common eiders or double-crested cormorants both with three to six eggs
per clutch (Drury 1973).
If the species that nest in colonies show a high degree of site
tenacity, they are not likely to reestablish a colony after it has
been eliminated. An exception to this is the food subsidy provided
by man, which seems to have been important in creating a nonbreeding
population of herring gulls large enough to form a "critical mass" for
the formation of a new gullery.
_Age Structure_
Because the main element of population size--the number of breeding
adults--is limited by the number of breeding colonies and the food
available to those colonies, one assumes that the total numbers of
seabirds is much less than could be supported by the larger areas
of productive oceans. Hence one suspects that there is lessened
competition for food outside the breeding season and that lack of
competition for food is a major reason for seabirds being long-lived,
often to extremes little suspected until recently. Mortalities of
10-12% per year are common, and some as low as 4% (wandering albatross,
_Diomedea exulans_; Tickell 1968) have been recorded.
In contrast, songbirds with large clutches, such as the titmice studied
by Kluyver (1951), produce a large number of young with whom they and
other adults must compete for food during the winter period of food
shortage. Because the titmice are permanent residents, they occupy all
of the available habitat throughout the year. Hence titmice suffer
intense intraspecific competition, which shortens the survival of
adults. Kluyver's experiments (1966) with nest boxes used by a closed
population of great tits on Vlieland, The Netherlands, showed that by
artificially reducing clutch size the survival of adults was increased.
Similar competition for the few territories available on marshes and
consequent shortened life expectancy, can be expected in waterfowl with
large broods. The effect should be less marked for geese with smaller
clutches that nest in less confined habitats.
The long life span of seabirds means that a population will have a
large component of older age categories; this characteristic has
several implications:
• It means that the population can survive years of reproductive
failure without the observable immediate effects that would be
manifest in titmice, grouse, or rabbits. Near failure of reproduction
during a breeding season among arctic seabirds at Bear Island was
reported by Bertram et al. (1934). Many similar observations have been
made since then: Pitelka et al. (1955) reported such a case among skuas
and gulls at Point Barrow, Drury (1961) for greater snow geese _(Chen
cerulescens atlantica)_ at Bylot Island, Jones (1970) for black brant
gathering at Isambek Lagoon on the Alaska Peninsula, and D. A. Snarski
(personal communication) for kittiwakes at Cook Inlet. Reproductive
failure can sometimes be chronic, as observed by Nisbet (1972) for
terns at Cape Cod, Massachusetts, or by Drury (1963) and Hunt (1972)
for herring gulls on the outer islands on the coast of Maine.
When reproductive failure becomes chronic as observed on peregrine
falcons _(Falco peregrinus)_ by Hickey (1969) and in ospreys _(Pandion
haliaetus)_ by Ames and Mersereau (1964), the population of adults may
hold on for a number of years without evident decline. Damage to the
structure of the whole population may be serious before any numerical
results are evident.
• Although there may not be intensive competition for food in the
habitat away from breeding colonies, there is intense competition for
food and breeding sites at and around the colonies. Hence age and
previous experience in seabirds assume importance in establishing
territory and in breeding success. Associated with this is the tendency
for immature birds to delay breeding until they are several years old
and for the immatures to remain on feeding grounds at some distance
from the colonies. In some cases young birds may "hang around" breeding
colonies and even feed some of the young. When young birds do first
breed they usually lay smaller clutches and raise fewer young than do
older birds. The importance of age and experience upon breeding success
has been well documented for kittiwakes (Coulson 1966) and red-billed
gulls (Mills 1973).
The fundamental biological importance of this delayed maturity seems
to be emphasized by the persistence for several years of immature
plumages, so clearly identifiable that even a human observer can
recognize the age of an individual. One assumes such an evident feature
must have adaptive significance.
_Wintering Grounds_
When colonial nesting seabirds leave their breeding islands for their
wintering grounds, their identification with that island is lost as far
as population effects are concerned, because birds from many colonies
mingle on the wintering grounds. Major mortality takes place on the
wintering grounds and must therefore act on the species population
as a whole rather than differentially on individuals associated with
especially dense colonies. Such a direct relation between colony
density and mortality would be necessary for density-dependent
mortality to regulate the number of birds on a breeding colony.
Conversely, one cannot expect that all colonies will decrease equally
because mortality should be equally distributed if all the population
gathers on a common wintering ground. Thus density-dependence acts only
in a very general way upon the sum of animals considered as an abstract
entity--the population.
In fact, on the wintering grounds, as shown by a graph of numbers of
gulls reported on Christmas Counts on Cape Cod, Massachusetts (Kadlec
and Drury 1968), herring gulls are very responsive to local conditions
and move several tens of miles to gather at favorable feeding sites.
An aerial survey of the gulls on the East Coast of the United States
(Kadlec and Drury 1968) showed that more than half of the gulls were
gathered near major food sources in large metropolitan districts.
Most of the remainder were gathered near small fishing ports. Very
few were scattered along the shoreline in what one assumes is the
traditional gull habitat. Later analyses of the relation between the
distribution of banding recoveries of birds in their first winter and
the distribution of immatures as found on this winter census (Drury and
Nisbet 1972) suggested that proportionately more first-year gulls died
in those areas where the birds were sparsely distributed than died in
the crowded metropolitan areas.
These results suggest both that there is not a direct feedback
between reproductive rate and mortality, and that mortality may even
be inversely density-dependent on wintering grounds. This last runs
counter to traditional ecological ideas that density causes a change
in mortality rate. The idea that individuals gather where "living is
easy" and mortalities are low is consistent with the theory of natural
selection. One would not expect the food of the gulls to be evenly
distributed, and one would expect individuals to move away from areas
where food is scarce and mortality is high.
_Differences in Breeding Success Between Colonies_
Breeding success has been shown to vary among individual pairs of
gulls (Drost et al. 1961). Certain groups of individuals nesting in
patches within a single colony have greater breeding success than do
others (Coulson 1968; Drury and Nisbet, in preparation). Differences
in breeding success also occur between colonies (Frazer-Darling
1938; Kadlec and Drury 1968; Drury and Nisbet 1972). Some colonies
reproduce consistently better than others--for example, the gull
colonies close to fishing ports and metropolitan areas. Other colonies
produce consistently fewer young, such as the colonies on the outer
islands in the Gulf of Maine (Drury 1963; Kadlec and Drury 1968; Hunt
1972). The populations of successful colonies grow while the numbers
of unsuccessful colonies decline, even during a period of general
population increase (Kadlec and Drury 1968).
The difference between success and failure, growth and decline,
appears to lie in the food available. Colonies increase where breeding
success is high and decrease where breeding success is low. One
important reason seems to be that adult gulls may move to a more
productive colony even after they have nested with another colony
(Drury and Nisbet 1972; Kadlec 1971). Such adaptations can be viewed
as adjustments by which individuals meet the requirements of an
environment in which the availability of food and other necessities is
patchy and shifting.
_Dispersal_
In general terms, the willingness of some individuals to disperse while
the majority of individuals remain loyal to a colony can be considered
a major mechanism of population maintenance. If conditions deteriorate
seriously at one place so that the local populations decline or
disappear, dispersal from other centers can be expected to repopulate
the area as soon as local conditions again become suitable. This
subject has been treated in more detail by Drury and Nisbet (1972) and
Drury (1974_b_).
Occupation of new, or return to former, nesting sites has been recorded
in detail for fulmars _(Fulmaris glacialis)_ by Fisher (1952) and for
herring gulls by Kadlec and Drury (1968). Dispersal is also known for
waterfowl. Hansen and Nelson (1957) reported that of some 8,000 brant
banded in midsummer on the Yukon delta 8 were recovered in northern
Siberia and 28 in northern Alaska and arctic Canada. They suspected
that pairing on the wintering grounds was responsible for the change in
breeding areas, a change that would not be expected among other North
American species of geese. Similarly, wide dispersal seems to occur
in pintails _(Anas acuta)_, mallards _(Anas platyrhynchos)_, and wood
ducks _(Aix sponsa)_.
The general tendency for some individuals to disperse and the frequency
of "extra limital" breeding attempts is especially well established in
the Bering Sea region, in part at least because vagrants from Siberia
or North America are readily identified as such. In the Aleutian
Islands, Emison et al. (1971) and Byrd et al. (1974) have enumerated
the nesting vagrants. For the Pribilof Islands, Kenyon and Phillips
(1965), Sladen (1966), and Thompson and DeLong (1969) have recorded the
repeated appearance of birds of Siberian distribution, and Fay and Cade
(1959) and Sealy et al. (1971) did the same for St. Lawrence Island.
One can conclude that a few individuals are constantly trying to
settle in new geographical areas. As climatic and habitat conditions
change, some populations are able to become established; for example,
southern species such as mockingbirds _(Mimus polyglottus)_, cardinals
_(Cardinalis cardinalis)_, and tufted titmice _(Parus bicolor)_ have
settled in southeastern New England during the last 2 decades. These
southern species have received much publicity. But at the same time,
a less publicized dispersal of white-throated sparrows _(Zonotrichia
albicollis)_, hermit thrushes _(Catharus guttatus)_, and dark-eyed
juncos _(Junco hyemalis)_ has resulted in new nesting records of more
northerly species, also in southeastern New England.
The ability (or lack of ability) of some organisms to expand their
ranges over time has been a subject of consideration for a number of
years by plant and animal geographers. An important botanical paper on
this subject in the Bering Sea region was presented by Hultén (1937),
who analyzed the ranges of plants of the area of Kamchatka, eastern
Siberia, Alaska, and northwest Canada, showing that diverse floras
occur in some restricted geographic areas. He called these areas
"refugia," and postulated that many species had survived Pleistocene
glaciations in them because these refugia remained ice-free. He, like
Fernald (1925), was puzzled as to why only certain species had been
able to expand their ranges outward from these "areas of persistence,"
while other apparently more "conservative" species were unable to do
so. Similarly, there appear to be conservative endemic bird species
of the Bering Sea region: the extinct Commander Islands cormorant
_(Phalacrocorax perspicillatus)_, Steller's eider _(Polysticta
stelleri)_, spectacled eider _(Lampronetta fisheri)_, emperor goose
_(Philacte canagica)_, whiskered auklet _(Aethia pygmaea)_, least
auklet _(A. pusilla)_, parakeet auklet _(Cyclorrhynchus psittacula)_,
Aleutian tern _(Sterna aleutica)_, red-legged kittiwake _(Rissa
brevirostris)_, bristle-thighed curlew _(Numenius tahitiensis)_,
long-billed dowitcher _(Limnodromus scolopaceus)_, surfbird _(Aphriza
virgata)_, black turnstone _(Arenaria melanocephala)_, rock sandpiper
_(Calidris ptilocnemis)_, and western sandpiper _(C. mauri)_.
The ranges of horned puffins _(Fratercula corniculata)_, Kittlitz's
murrelet _(Brachyramphus brevirostris)_ and, perhaps, crested auklet
_(Aethia cristatella)_ suggest that some species of "Beringian"
seabirds have expanded their ranges from Hultén's (1937) "refugia."
_Dispersal and Regional Persistence of Marginal Populations_
The presence of several sub-elements of a species population and,
therefore, the opportunity for dispersion among alternative breeding
sites may be an important factor in the regional persistence of a
species on the margin of its range, as illustrated by the history of
laughing gulls _(Larus atricilla)_ in New England.
Between 1875 and 1900 there were fewer than 50 laughing gulls in
Massachusetts (Mackay 1893) and about 35 in Maine (Norton 1924). In
Massachusetts the laughing gulls all settled on one large island,
Muskeget, where by 1940 there were about 20,000 pairs (Noble and Würm
1943). Meanwhile, in Maine the population had been disturbed by sheep
and men and had shifted about among seven islands. The Maine population
grew to only about 350 pairs by 1940 (Palmer 1949).
The laughing gull population in both States has decreased since 1940.
In Massachusetts, where all pairs occupied one island, the population
had fallen to about 250 pairs by 1972, but the Maine population, still
divided into five colonies, stabilized at 250 pairs (i.e., the same as
instead of only 1% of the Massachusetts population).
Use of General Principles in Solving Conservation Problems
Game biologists have successfully maintained the populations of hunted
animals by using a number of classical principles of game management.
They have controlled mortality by regulating kill and have increased
standing stock by improving habitat on a local scale. This seems to
have worked in species which are short-lived, have large clutch sizes
or litters, and which occupy mosaics of highly productive "successional
habitat." Seabirds, however, contrast with these species in a number
of important biological characteristics. They have small clutches,
postpone breeding until they are several years old, and are subject to
periodic or chronic reproductive failures. Therefore, their populations
are skewed toward older animals and replacement of lost individuals
is slow. Many seabirds, like some geese, have a high level of site
tenacity and thus may resist recolonization or fail in the attempt to
recolonize a breeding site once eliminated from it. In those species
studied it appears that the breeding birds at a small percentage of
colonies are responsible for a large proportion of the annual crop of
young. It is probably dangerous, therefore, to risk either damage to or
elimination of well-established colonies.
Studies of kittiwakes by Coulson and White (1958, 1961), sooty terns
_(Sterna fuscata)_ by Ashmole (1963) and Harrington (1974), Atlantic
puffins by Nettleship (1972), and Cassin's auklets _(Ptychoramphus
aleutica)_ by Manuwal (1974), and the practice of eider "farming" in
Iceland indicate that the number of available territories or breeding
sites may limit the size of a population and that populations can be
increased by increasing the number of sites available. This suggests
one way in which direct steps can be taken to encourage the numbers of
breeding seabirds. Other studies indicate that seabirds will move into
synthetic habitat such as created by the window ledges on buildings
(Coulson and White 1958) or the islands created by dumping spoil from
channel dredging operations (Buckley and Buckley 1971, 1975; Soots and
Parnell 1975).
Most generalizations of population biology have been derived from the
study of insects, songbirds, or game species. It seems inadvisable
to assume that those principles will apply to seabirds without
modification. For example, predation by gulls and ravens may have a
disastrous effect on a seabird colony at low colony density but have
progressively less impact as the colony size and density increase. Fox
predation may have important effects over most ranges of prey density
because the presence of foxes has important psychological effects.
The habitats of seabirds include elements in which birds are widely
dispersed (feeding areas) and others in which birds are crowded
and narrowly localized (nesting sites). Thus effective programs of
conservation should include guarantees that a number of colony sites be
available in as widely dispersed a pattern as possible. Each productive
feeding ground should, if possible, have several colony sites available.
We have argued elsewhere (Drury and Nisbet 1972; Drury 1974_a_) that
one of the chief defenses any population has against extinction is
the combination of being divided into a number of population centers
with having some movement of individuals between the centers, but not
too much. Because it is highly improbable that a single catastrophe
will affect more than a part of a species' range at any time, the more
numerous and widely scattered the partially independent segments of a
population are, the better the species is insured against extinction.
This, of course, suggests that the size of each colony may be less
important for long-term survival than is the total number of colonies.
One intuitively concludes that "conservative" species, such as those
endemic to the Bering Sea region (whose dispersal and colonizing
mechanisms seem to be poorly developed), are especially vulnerable
to the effects of local population crashes. These "local" species
therefore deserve special consideration.
I would like to emphasize two points to be included in designing a
"management" program:
• It seems that the most promising management techniques will be built
upon ensuring the health of colonies and the associated feeding areas
at which reproductive success is high enough to "export" young. Thus
it is useful to identify those colonies which are exporting young and
to give special care to their preservation. As populations of prey
species change locally, so will the success of the local nesting birds.
A colony which is thriving at one time may be barely maintaining itself
at another (Ainley and Lewis 1974), or it may decrease, as in the case
of "guano birds" during El Niño years in the Peru Current.
• Because centers of abundance of marine birds shift (Fisher 1952;
Drury 1963, 1974_a_), it will be prudent to plan for large areas
and over long periods of time. Harrison Lewis, a pioneer in seabird
management in eastern Canada, said (personal communication) that just
as soon as he got approval of a new seabird sanctuary through the long
corridors of the distant government bureaucracies in Ottawa, the birds
would move to a new island and he had to start the process all over
again.
The objective is to maintain a variety of colony sites for populations
to move among as local patterns of productivity in the shallow sea
shift.
Goals for Research on Population Dynamics of Seabirds for Purposes of
Conservation
1. To learn the distribution and relative importance of seabird
colonies, the number of pairs nesting and nonbreeding individuals
at each colony, and the timing of breeding activities for each
geographical region. The most important step toward conserving marine
birds is to get public ownership and protection for their breeding
grounds.
2. To understand the life cycle of key species. Three needs are clear:
a. To identify key species whose biological characteristics can
conveniently be studied and measured. Studies of these species may be
useful in monitoring the "health" of seabird breeding areas.
If it is established that the reproductive success of certain species
varies similarly in response to changes in their marine habitat (such
as black-legged kittiwakes and horned puffins), one could use key
species (black-legged kittiwake) to assess the performance of those
species in a colony whose breeding success is difficult to measure
(horned puffin).
b. To develop efficient and practical ways of censusing and measuring
productivity of crevice-, scree-, and hole-nesting species such as
puffins and auklets.
c. To establish annual differences in reproductive success and
mortality rates by age classes of the key species, and from these to
identify rates of population turnover so as to be able to predict the
effects of mass mortalities.
3. To learn enough about the differences in behavior and productivity
among colonies to establish which colonies produce surplus young and
which have low productivity. At first, maximum efforts for conservation
should be concentrated at those sites which produce surplus young.
4. To learn about colonial behavior. Two needs are apparent:
a. To know enough about the lives of individually marked birds of known
age so as to be able to infer the behavior of population elements at
all stages of their life cycle.
b. To know enough about the lives of subadult birds to understand
what proportion of subadults visit and become established at breeding
sites, why the subadults visit the breeding sites and what effect their
presence has on the territories and breeding success of their neighbors
and biological relatives.
5. To know enough about places where seabirds, waterfowl, and
shorebirds gather on migration and during the winter to identify
those areas which need special protection from effects of economic
development.
a. It is important to determine the areas where marine birds gather at
sea when they are away from their breeding grounds. What factors of
habitat and food supply make certain places preferable to others? What
is the relation between gathering grounds and underwater topography
(banks and edges of the continental shelf)? What are the seasonal and
annual differences in preferred gathering grounds? What special hazards
exist, such as unusual extent of sea ice or exceptional storms?
b. It is important to plot coastal areas where waterfowl and shorebirds
gather on migration, for molting, and during the winter. Which open
leads in the ice and patches of open water at the mouths of rivers
are of especial importance in spring? What shorelines and beaches act
as "leading lines" during migration? Which capes and points result
in concentrated overflights of migrating waterfowl, and hence are
locations of unusually high kills by hunters? What wetlands, bogs,
coastal ponds, lakes, and lagoons are used as gathering grounds and
to what extent do waterfowl and shorebirds exchange between gathering
grounds? How much redundancy of wetlands is needed to make the wetlands
system maximally productive for waterfowl and shorebirds?
Answers to these questions will identify which geographic areas deserve
special protection during development. The answers will also identify
the kinds of influences which might lower the contribution of each
critical area to the survival of seabirds, waterfowl, and shorebirds.
Areas identified as important under these categories must be included
in policy decisions related to land-use planning and management.
6. To learn more about the effects of varying quantities of food on
breeding behavior and performance:
a. What are the effects of food abundance in early spring on date of
laying, clutch size, and egg size?
b. What effects do storms have at different stages of the reproductive
schedule?
c. What effects do quantitative and qualitative (species composition of
prey) changes in food supply have on the survival of chicks?
d. What are the similarities and differences between what parents eat
and what they feed their chicks?
Although this is important basic biological knowledge, it contributes
little to conservation efforts because food differences result from
changes in the ocean over which humans can have little effect.
7. To learn more about prey species and their availability to marine
birds:
a. To know more about the breeding areas, reproductive rates, growth
rates, and routes of dispersal of the major prey food species. In most
areas a few species of teleost fish (e.g., _Ammodytes_) or Crustacea
(e.g., copepods, euphausids, mycids, or amphipods) make up most of
the food of marine birds. Yet, the barest minimum is known about the
biology of such species. A good first estimate of the "condition" of
the marine environment can probably be made by measuring reproductive
rates and growth rates of these key prey species. Hence an efficient
(though indirect) way to measure those rates may be by monitoring
reproduction of birds.
b. To know more about the density and distribution of key prey items
season by season, and to learn more about the relation of their
abundance and distribution to their availability to birds, as Bédard
(1969) showed for _Calanus_ to least auklets, and _Thysanoessa_ to
crested auklets.
There are some indications that the population size of prey items can
vary widely without having a marked effect on the numbers of their
predators. Does commercial fishing for the large, predatory fish have
a measurable effect on the food available to marine fish? Do the large
pollock and salmon fisheries (high seas) make zooplankton available to
smaller alcids? Do marine birds affect a fishery?