Part 14

Birds which reached Micronesia by way of the islands of Celebes and the Moluccas may have been derived originally from Melanesia. The following birds appear to have used this route: Porphyrio porphyrio, probably Halcyon chloris, Rhipidura lepida, Myiagra oceanica, Zosterops conspicillata, and Z. cinerea. These birds apparently became established initially in the Palaus; Porphyrio and Rhipidura lepida have not been recorded elsewhere in Micronesia, but Myiagra and the two species of Zosterops have spread to the Carolines and Marianas, although not into the Marshall Islands. Wind from the southeast in summer and fall has probably been a factor aiding these colonizations. The population of Gallinula chloropus resident at Palau may also have arrived by this route.

PHILIPPINE COMPONENT

Ten of the kinds of birds of Micronesia have come from or by way of the Philippine area. These are known principally from the Palaus and the Marianas and include: Rallus philippinus, R. owstoni, Poliolimnas cinereus, Caprimulgus indicus, Corvus kubaryi, Psamathia annae, Artamus leucorhynchus, possibly Lonchura nigerrima, and Collocalia inexpectata. The Philippines may have been the actual point of dispersal of the birds (example, Psamathia), or may have been used as a stepping stone to Micronesia by birds coming from Melanesia (examples, Rallus and Artamus), by birds from Malaysia (example, Collocalia), and by birds from Asia (example, Caprimulgus). Two birds of this component have reached the islands of eastern Micronesia. A subspecies of Lonchura nigerrima is endemic at Ponapé, and a subspecies of Poliolimnas cinereus occurs on several islands in the Carolines and has even been recorded at Bikini in the Marshall Islands. Three species are known only from the Palaus; two are known only from the Marianas.

PALEARCTIC COMPONENT

Birds of Micronesia which have been derived directly from Palearctica are Gallinula chloropus guami, Otus podarginus, Asio flammeus, Acrocephalus luscinia and Anas oustaleti. Apparently Gallinula, Asio, and Acrocephalus arrived in Micronesia by way of the chain of islands from Japan southward to the Bonins, Volcanoes, and Marianas. Otus reached Palau from Asia, possibly by way of the Philippines. The smallness of the representation of this component may result partly from lesser ability of the northern birds to adapt themselves to, and to establish themselves on, the semi-tropical and tropical islands of Micronesia as compared with birds from Melanesia where the climate and ecologic conditions resemble more closely those found in Micronesia. Evidence supporting this possibility is the large number of Palearctic residents in the Bonin and Volcano islands as compared with fewer in the Marianas; the Bonins and Volcanoes are less tropical and more temperate in climate.

Table 11 lists the birds concerned, by faunal areas from which the birds have been derived and shows the number of kinds of birds which are present as a result of these colonizations. There is some overlap in the numbers since some endemics may be found in more than one area in Micronesia. Figure 8 shows the faunal areas from which the endemic land and fresh-water birds of Micronesia have been derived. Melanesia (Papua) supplied 52 percent of this population. Birds reaching Micronesia by way of the Moluccas and Celebes include 21 percent of the total population. The Philippines have supplied 10 percent; Polynesia, 9 percent; and Palearctica, 8 percent. This population of endemic land birds and fresh-water birds has seemingly evolved from 46 colonizations, of which 27 have been derived from Melanesia, 6 from the Philippines, 5 from the Moluccan and Celebean areas, 5 from Palearctica, and 3 from Polynesia.

The Palaus have received a large part of their avifauna from the west (Moluccas, Philippines, Palearctica). Their Melanesian component is mostly the result of secondary colonization from the Carolines. The Carolines have received a greater share of their land birds and fresh-water birds from Melanesia and a smaller share from Polynesia. The Marshalls are definitely associated with the Polynesian element. The Marianas exhibit a considerable amount of secondary colonization from other Micronesian islands, as well as some unique components from the Philippines, Melanesia, and Palearctica. Thus, the number of endemics in Micronesia provides little information concerning the actual number of successful colonizations by birds from other areas. Many of the endemics probably have resulted in this way: Individuals of an endemic subspecies flew to another island and there underwent further differentiation, producing another endemic subspecies. Such secondary colonization probably is going on now.

This analysis of the avifauna shows that Micronesia, with the exception of the Marshall Islands (and the Gilbert Islands), has but little affinity to Polynesia. It has greater affinity, from the zoogeographical standpoint, with the Papuan Region (Melanesia).

TABLE 11. AVIFAUNAL COMPONENTS WHICH MAKE UP THE ENDEMIC RESIDENT LAND AND FRESH-WATER BIRD POPULATION OF MICRONESIA

=================+=======+=============+===========+==========+========== | | Western | | | FAUNAL COMPONENT | Palau | and central | Eastern | Marianas | Marshalls | | Carolines | Carolines | | -----------------+-------+-------------+-----------+----------+---------- Polynesian | 2 | 3 | 5 | 0 | 3 Melanesian | 11 | 14 | 16 | 12 | 0 Moluccan-Celebean| 6 | 3 | 4 | 7 | 0 Philippine | 6 | 2 | 2 | 4 | 1 Palearctic | 2 | 1 | 2 | 5 | 0 +-------+-------------+-----------+----------+---------- Totals | 27 | 23 | 29 | 28 | 4 -----------------+-------+-------------+-----------+----------+----------

SPECIATION

Of the 104 native fresh-water birds and land birds which are resident in Micronesia, only 7 kinds or 6.5 percent remain undifferentiated from populations elsewhere. These birds are Phalacrocorax melanoleucus, Pandion haliaetus, Demigretta sacra, Ixobrychus sinensis, Anas poecilorhyncha, and possibly Lonchura punctulata (may be an introduction by man). Another bird, Gallinula chloropus, a resident at Palau, may or may not be distinct from the gallinule of Malaysia, G. c. orientalis. Of the 104 resident birds, 97 kinds or 93.5 percent have become differentiated and can be separated taxonomically from populations elsewhere. Of the kinds of birds which are found only in Micronesia, there are 5 endemic genera (16 percent), 31 endemic species (32 percent) and 76 endemic subspecies (75 percent). If we consider the avifauna of Micronesia as a single element, the endemism is high as compared with that on larger and less isolated islands. For example, Mayr (1944a:174) found 137 resident birds on Timor including 22 endemic species (16 percent) and 67 endemic subspecies (47.5 percent). Stresemann (1939b:313) found 220 species including 84 endemic species (38.2 percent) on Celebes. Mayr (1944a:174) also writes that on Java, of 337 breeding species, 16 (4.8 percent) are endemic, and on New Caledonia, of 68 species 19 (27.9 percent) are endemic. Speciation in Micronesia has not progressed much farther than that at New Caledonia and not so far as at Celebes, but subspeciation has progressed considerably more than at the island of Timor. The avifauna of the Hawaiian Islands, as recorded by Bryan and Greenway (1944), has 73 resident land birds and fresh-water birds, all of which are endemic, including one family, 23 genera and 36 species. The North American night heron, Nycticorax n. hoactli, may be included in this list as the only resident which is undifferentiated. The development of full specific differentiation within the resident avifauna is greater in the more isolated Hawaiian chain where 49 percent of these birds are regarded as endemic species, while in Micronesia, which is less remote from other bodies of land, the specific endemism is only 32 percent.

TABLE 12. ENDEMISM IN FAMILIES OF NATIVE LAND AND FRESH-WATER BIRDS IN MICRONESIA

==================+===========+=========+=========+============+========= | | Endemic | Endemic | Endemic | Total FAMILY | Residents | genera | species | subspecies | 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 | 2 | 0 | 1 | 1 | 2 Caprimulgidae | 1 | 0 | 0 | 1 | 1 Apodidae | 5 | 0 | 1 | 5 | 5 Alcedinidae | 7 | 0 | 1 | 7 | 7 Campephagidae | 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.

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:

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.