The avifauna of Micronesia consists of 206 kinds of birds belonging to
37 families and 91 genera. Of these, 30 kinds are sea birds, 29 kinds
are migratory shore birds, and 146 kinds are land and freshwater birds.
Of the 30 sea birds, 18 kinds are resident; of the 147 land and
fresh-water birds, 104 kinds are resident and 6 kinds have been
introduced by man. There are no resident shore birds in Micronesia. The
following conclusions can be drawn from this study:
1. The islands of Micronesia are oceanic islands and were seemingly
formed independently of any present day continental land mass.
Terrestrial organisms have reached these islands by "over-water
dispersal." The avifauna of Micronesia has been received from the
following sources: Polynesia, Melanesia, the Moluccas, Celebes,
Phillipines, and Palearctica (see figure 8).
2. Oceanic birds are among the oldest forms of bird life inhabiting
Micronesia. The presence of elevated islands containing phosphate,
resulting from the deposition of guano by oceanic birds, is some
indication of the length of time during which these birds have been
present. In number of individuals, the oceanic birds inhabiting the
inshore zone are more numerous than those inhabiting the offshore and
pelagic zones, although twelve of the eighteen resident kinds of oceanic
birds prefer the offshore and pelagic zones. Most of the species of
oceanic birds resident in Micronesia are circumtropical in distribution;
no residents are known in Micronesia which have been derived from
Palearctica or the North Pacific. Micronesia has no endemic oceanic
birds.
3. On the migratory flights, shore birds reach Micronesia along three
distinct flyways, which in this report are named the Asiatic-Palauan
Flyway, the Japanese-Marianan Flyway, and the Nearctic-Hawaiian Flyway
(see figure 7). The shore birds began to utilize the Pacific islands as
wintering grounds by gradually spreading from the Eastern Hemisphere
rather than from the Western Hemisphere.
4. More than half (52 percent) of the land birds and fresh-water birds
in Micronesia were derived directly from ancestral stocks in Melanesia.
The areas of the Moluccas and of Celebes (Malaysia) supplied 21 percent
of the birds; the Philippines, 10 percent; Polynesia, 9 percent; and
Palearctica, 8 percent. Results of this study show that there may have
been only 46 actual colonizations of Micronesia by birds from other
areas, and that many of the large number of endemics present have been
the result of secondary colonizations within the islands of Micronesia.
It is concluded that Micronesia, except for the Marshall Islands, has a
much closer affinity to Melanesia than to any other area as regards
avifauna. The Marshall Islands may be regarded as a part of the
Polynesian Subregion from the viewpoint of avian zoogeography.
5. Endemism in the land birds and fresh-water birds of Micronesia is
extreme. Of 104 native, resident birds, 97 (93.5 percent) have become
differentiated and can be separated taxonomically from related forms. In
Micronesia, there are 5 endemic genera, 31 endemic species, and 76
endemic subspecies. The families containing the greatest number of
endemic forms are Muscicapidae (14), Zosteropidae (14), Columbidae (13),
and Sturnidae (9).
6. It is concluded that some of the more important factors controlling
the dispersal of the bird life to Micronesia are the direction and the
intensity of the winds, the small size of the islands, the isolation of
the islands (especially those "high" islands), and the insular
climates, which appear to favor colonists from tropical homes rather
than those from Palearctic homes.
7. The factors most important in the process of differentiation of birds
in the islands of Micronesia are isolation, paucity in numbers of
individuals, freedom from predation, absence (and presence) of
interspecific and intraspecific strife, and nutrition. The importance of
the "dilution" factor is discussed, and the possibility of
cross-breeding between different kinds of birds is considered. It is
concluded that genetic change altering the phenotypic expression of
avian characteristics is no more apt to occur in insular populations
than in continental populations, but such changes have a greater chance
of being perpetuated in insular populations.