Darlington (1938:274) in discussing the origin of the fauna of the
Greater Antilles uses the term "over-water dispersal" in referring to
the spread of terrestrial animals across water. He is against the use of
the term "accidental dispersal" since many factors besides accident are
involved. He contends, as do others, that certain forms of organisms,
owing to their "nature and behavior" cross water barriers more
successfully than others. These observations may be applied to the
"over-water dispersal" of birdlife to the islands of Micronesia. Certain
groups of birds are more evident in Micronesia than others. Certain
groups of birds which are found on other islands of the Pacific basin
are found in Micronesia only in small numbers or may not be
represented; Mayr (1945a:284) writes, "Remarkable is the almost complete
absence of parrots and honey-eaters, the small number of pigeons and the
absence of such widespread genera as _Lalage_, _Turdus_, and
_Pachycephala_." The absence of some species and the presence of others
produces the characteristic insular effect termed "disharmonic" by
Gulick (1932:407), as compared with the continental area or island which
derived its avifauna by way of a land bridge. One would think from
looking at table 12 that members of the families Rallidae, Columbidae,
Muscicapidae, Sturnidae, and Zosteropidae were the most successful
colonizers in Micronesia on the basis of the number of successful
colonizations (not necessarily on the number of endemics developed from
a single colonization). Of these families, Sturnidae and Zosteropidae
and possibly Columbidae contain species which often move in flocks.
Furthermore, these families as well as the Muscicapidae feed on either
fruits, seeds, or insects, any one of which is a type of food which
might "give out" suddenly, stimulating a migratory behavior within the
birds. From a flock embarking seaward in "search" of more food, a part
or even all of the birds might survive in a chance flight to an isolated
island in Micronesia. If a flock containing both males and females
reaches an island, the species has a good chance of becoming
established. Evidence that such a rapid colonization by flocks of birds
can take place is found in the remarkable colonization of New Zealand by
_Zosterops lateralis_ from the Australian area. The bird was first seen
as a winter migrant in New Zealand in 1856 and records of nestings were
obtained at North Island in 1862, according to Oliver (1930:489). In the
case of rails there is no evidence that they move in flocks; however,
they are among the most successful colonizers and are on many of the
oceanic islands in the tropical and subtropical oceans. Representatives
of several species of the family Rallidae have invaded Micronesia and
have successfully established 6, or possibly 7, "colonies."
Darlington (1938:274) further writes that "it is no accident that some
islands, because of their nature and position, the direction of winds
and currents, and the nature of the neighboring land, receive more
organisms than other islands do." Semper (1881:294) writes that the
distribution of flying creatures "must be in a great degree dependent on
the direction and strength of atmospheric currents." These statements
are applicable to the history of the avifauna of Micronesia. The
Caroline Islands, for example, present a "broad front" for wanderers
from the Melanesian islands. As mentioned previously, the prevailing
winds in the late spring, summer, and early fall are from the south,
southwest, and southeast and would favor bird flight to the northward
towards the Carolines. In addition, the breeding season of many of the
birds in Melanesia is from November to February, and in the spring and
summer, restless young birds seeking living space might fly seaward and
aided by the winds fly northward towards Micronesia. Adults, which may
have well-established home territories, may be less likely to attempt
such a movement.
One could conclude from the above discussion that the Micronesian
islands, especially the Carolines, might be well populated with a large
variety of birds from Melanesia, a scant 500 or more miles away. As it
turns out, there are only a few islands in this extensive archipelago
possessing proper vegetation, fresh water, and other qualities which
make them capable of supporting the land and fresh-water birds of
Melanesia. The few islands which have these qualities are the so-called
"high" islands, including the entire Mariana chain, the Palaus, and four
widely separated islands in the Carolines: Yap, Truk, Ponapé, and
Kusaie. The other islands of Micronesia are "low" coral islands, which
often lack fresh water and have a meager variety of fruits, insects and
other foods. Thus, if birds do reach Micronesia but arrive at the atolls
instead of the "high" islands, these birds may be doomed. It is
noteworthy that the Micronesian islands are small compared with the
Solomons, Fijis, and others. The smaller the island, the fewer the
number of ecologic niches and the fewer the kinds of birds present.
Mayr (1941b:215) writes that the distance from the nearest land mass and
the climatic conditions are important factors controlling dispersal.
With regard to the degree of remoteness of the islands, table 13 lists
the number of resident land and fresh-water birds present in the Palaus
and the "high" islands of the Carolines. Also, the approximate distance
from the nearest large land mass and the area in square miles are given.
There is some correlation between the distance from the nearest land
mass and the number of resident land birds and fresh-water birds. For
example, Palau, with 32 resident birds, is only 410 miles from the
nearest land mass whereas Kusaie, with only 11 resident birds, is 720
miles from the nearest land mass. The comparative size of the land mass
must also be taken into account, as shown by the fact that the large
island of Ponapé contains more kinds of birds but is more remote from
large land masses than either Yap or Truk.
TABLE 13. CORRELATION BETWEEN NUMBER OF RESIDENT LAND AND FRESH-WATER
BIRDS AND DISTANCE FROM LARGE LAND MASSES OF "HIGH" ISLANDS OF
MICRONESIA
Column headings:
A: No. of Birds
B: Approximate distance from nearest land mass (statute miles)
C: Area in square miles
========+====+=====+===================================+=====
ISLAND | A | B | Nearest land mass | C
--------+----+-----+-----------------------------------+-----
Palau | 32 | 410 | Approximately equal distance from | 171
| | | Mindanao, Morotai, New Guinea |
| | | |
Yap | 13 | 580 | New Guinea | 83
| | | |
Truk | 17 | 525 | New Ireland | 50
| | | |
Ponapé | 20 | 630 | New Ireland | 145
| | | |
Kusaie | 11 | 720 | Malaita (Solomons) | 42
--------+----+-----+-----------------------------------+-----
Climatic factors are important in the dispersal of bird life;
Micronesia, where the climate is tropical to subtropical, is better
suited for colonization by birds from the tropics (Melanesia) than by
birds from the temperate or cold climates (Palearctica). The climatic
factor may be one of the principal reasons why birds from Palearctica
make up only a small part of the avifauna of Micronesia.