Table 12. Endemism in Families of Native Land and Fresh-water Birds in Micronesia
| Family | Residents | Endemic genera | Endemic species | Endemic subspecies | Total endemic |
| Phalacrocoracidae | 1 | 0 | 0 | 0 | 0 |
| Ardeidae | 3 | 0 | 0 | 1 | 1 |
| Anatidae | 2 | 0 | 1 | 0 | 1 |
| Accipitridae | 1 | 0 | 0 | 0 | 0 |
| Megapodidae | 2 | 0 | 1 | 2 | 2 |
| Rallidae | 7 | 1* | 2 | 4 | 6 |
| Columbidae | 13 | 0 | 4 | 11 | 13 |
| Psittacidae | 1 | 0 | 1 | 0 | 1 |
| Strigidae | 1 | 0 | 1 | 1 | 2 |
| Caprimulgidae | 1 | 0 | 0 | 1 | 1 |
| Apodidae | 5 | 0 | 1 | 5 | 5 |
| Alcedinidae | 7 | 0 | 1 | 7 | 7 |
| Apodidae | 3 | 0 | 0 | 3 | 3 |
| Corvidae | 1 | 0 | 1 | 0 | 1 |
| Sylviidae | 5 | 1 | 2 | 4 | 5 |
| Muscicapidae | 14 | 1 | 6 | 9 | 14 |
| Artamidae | 1 | 0 | 0 | 1 | 1 |
| Sturnidae | 9 | 0 | 3[+] | 7 | 9 |
| Meliphagidae | 7 | 1 | 1 | 6 | 7 |
| Zosteropidae | 14 | 1 | 6 | 10 | 14 |
| Ploceidae | 5 | 0 | 0 | 4 | 4 |
| Totals | 104 | 5 | 31 | 76 | 97 |
* Aphanolimonasa is included but may be extinct.
[+] Aplonis corvinus is included but may be extinct.
* Aphanolimonasa is included but may be extinct.
[+] Aplonis corvinus is included but may be extinct.
[Table 12] lists the families of land birds and fresh-water birds which have resident members as part of the avifauna of Micronesia. It can be observed from the table that only two families are represented by no endemic kinds, several families are represented by one or two endemic kinds, and others are represented by as many as 14 endemic kinds. Endemism has reached its greatest development in the families Rallidae (6), Columbidae (13), Apodidae (5), Alcedinidae (7), Sylviidae (5), Muscicapidae (14), Sturnidae (9), Meliphagidae (7), and Zosteropidae (14). Generic endemism is greatest in the Sylviidae where one endemic genus occurs among 5 endemic species and subspecies (20 percent), in Rallidae one in 6 (17 percent), in Meliphagidae one in 7 (14 percent). Specific endemism is greatest in Psittacidae and Corvidae where the single representative of each family in Micronesia is considered specifically distinct (100 percent), in Megapodidae and Strigidae one in 2 (50 percent), in Muscicapidae and Zosteropidae 6 in 14 (43 percent) in Sylviidae 2 in 5 (40 percent), in Rallidae 2 in 6 (33 percent), in Sturnidae 3 in 9 (33 percent) in Columbidae 4 in 13 (31 percent). Subspeciation within species which are endemic in Micronesia has occurred in 8 families, occurring within two species in each of the families Columbidae and Zosteropidae and once in each of the families Megapodidae, Apodidae, Alcedinidae, Sylviidae, Muscicapidae, and Sturnidae.
In summary, the families of land and fresh-water birds found in Micronesia which have the greatest number of endemic forms are Muscicapidae (14), Zosteropidae (14), Columbidae (13), and Sturnidae (9). Speciation has occurred in the single representative of the families Psittacidae (Trichoglossus rubiginosus) and Corvidae (Corvus kubaryi). Where family representation is large, speciation has occurred most frequently, as in the Muscicapidae (6 in 14 = 43 percent), in the Zosteropidae (6 in 14 = 43 percent), and in the Columbidae (4 in 13 = 31 percent). Subspeciation has occurred in 8 families, in two species in the Columbidae and Zosteropidae and in one species in each of 6 other families.
Time of Colonization
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: