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.