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.
Factors Causing Dispersal
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
Table 13. Correlation Between Number of Resident Land and Fresh-water Birds and Distance From Large Land Masses of "High" Islands of Micronesia
| Island | No. of Birds | Approximate distance from nearest land mass (statute miles) | Nearest land mass | Area in square miles |
| Palau | 32 | 410 | Approximately equal distance from Mindanao, Morotai, New Guinea | 171 |
| Yap | 13 | 580 | New Guinea | 83 |
| Truk | 17 | 525 | New Ireland | 50 |
| Ponapé | 20 | 630 | New Ireland | 145 |
| Kusaie | 11 | 720 | Malaita (Solomons) | 42 |