Previously (and in the accounts of the species to follow), comments are
made concerning the subjects of _from where_ and _by what route_ the
various kinds of birds have arrived at Micronesia. The problem of _when_
these birds arrived is a difficult and usually unanswerable one.
Although geology provides some evidence on the relative age of the
islands, and although deposits of bird guano on now elevated coral
islands show that oceanic birds have inhabited these islands for a long
time, there is no evidence to show the time of the first colonization by
land birds. No fossil remains of land birds or fresh-water birds have
been found in Micronesia. The relative extent of differentiation in
color and structure, which has taken place between different birds,
offers one means for estimating the relative length of residence in the
area, provided all other factors are equivalent. Concerning the birds of
the Galapagos, Lack (1947:113) writes "That Darwin's finches are so
highly differentiated suggests that they colonized the Galapagos
considerably ahead of the other land birds." Evidence from this source
actually is of little value, because the speed of evolution is unknown
and its rate may be different in different species, even though they
live under the same circumstances. Dobzhansky (1941) says that evolution
is a modification of the genetic equilibrium, which, if true, may not
result in similar manifestations in different kinds of birds living
under the same conditions of life. Relative antiquity of the birds might
be ascertained by measuring their ecologic adaptations. The Guam Rail
(_Rallus owstoni_) and the Micronesian White-browed Rail (_Poliolimnas_)
can be examined in this way. _R. owstoni_ has the ability to live in
both brackish and fresh water swamps, as well as in the scrub and grass
of the uplands and in the virtually barren, rocky areas in the dense
jungles. _Poliolimnas_, on the other hand, appears to be restricted to
swampy areas in Micronesia. If the swampy areas were removed this rail
probably would become extinct. _R. owstoni_ appears to have been
resident in Micronesia longer than _Poliolimnas_. However, ability to
live in a variety of habitats might be acquired by _R. owstoni_ in a
relatively short time.
Another possibility is that the birds, which are less differentiated
from their ancestral stocks, may be less differentiated because of
suppression of newly evolved characters by dilutions, which result from
interbreeding with new birds, which may be arriving at irregular
intervals from the ancestral home. Interbreeding of the resident
population with newcomers may overshadow any modifications which might
have appeared as a result of insular isolation, especially modifications
which have little adaptive significance. One would suspect, from their
modifications, that _Rallus owstoni_, _Metabolus rugensis_, _Corvus
kubaryi_, and other endemic forms have experienced less of this
"dilution," than such birds as _Rallus philippensis pelewensis_,
_Artamus leucorhynchus pelewensis_, _Myzomela cardinalis_, and others.
Murphy (1938) mentions this "dilution" effect in his discussion of
"strong" and "weak" subspecies among warblers of the Marquesas. He
writes that "strong" subspecies may develop if the birds are present on
islands which are upwind from islands containing related subspecies. The
wind acts to block interisland migration in these weak-flyers. On the
other hand, "weak" subspecies may show the effect of "dilution," being
situated on islands downwind from islands containing related
subspecies. The direction of the wind acts to aid the weak flyers to
move to the downwind islands and continually "dilute" the resident
subspecies. Similar examples can be cited for Micronesian birds. Hesse,
Allee, and Schmidt (1937:87) write, "Endemism on islands is most
frequent in forms for which the difficulty of reaching the island is
most extreme, so that new increments of the parent form are unlikely to
follow."
Employing the criteria mentioned above, the birds of Micronesia can be
tentatively divided into four groups as regards the relative time when
they arrived at the islands:
1. Birds of ancient colonizations which reached certain individual
islands, became modified, and dispersed no farther. Examples are
_Aphanolimnas_, _Rallus owstoni_, _Aplonis corvinus_, _Metabolus
rugensis_, and _Corvus kubaryi_.
2. Birds of ancient colonizations which reached or dispersed through a
number of islands but are now restricted to relatively few islands.
Examples are _Ducula oceanica_, _Ptilinopus porphyraceus_, _Megapodius
lapérouse_, _Asio flammeus_, and _Acrocephalus luscinia_.
3. Birds of ancient, or possibly more recent, colonizations which
initially reached or subsequently dispersed to many of the islands of
Micronesia possessing habitat suitable for them. Examples are _Myzomela
cardinalis_, the two species of _Halcyon_, _Aplonis opacus_, and
_Zosterops conspicillata_.
4. Birds of rather recent colonizations, which may have reached only a
few islands and are relatively unmodified from their parental stocks.
Examples are _Artamus leucorhynchus_, _Caprimulgus indicus_,
_Poliolimnas cinereus_, and _Nycticorax caledonicus_.