Part 11

TABLE 2. LIST OF RESIDENT AND NONRESIDENT OCEANIC BIRDS OF MICRONESIA

-------------------+----------+------------- Genera | Resident | Nonresident | kinds | kinds -------------------+----------+------------- Diomedea | 0 | 1 Puffinus | 4 | 1 Pterodroma | 1 | 1 Phaëthon | 2 | 1 Sula | 3 | 0 Fregata | 1 | 1 Larus | 0 | 1 Chlidonias | 0 | 1 Sterna | 2 | 4 Thalasseus | 1 | 0 Procelsterna | 0 | 1 Anoüs | 2 | 0 Gygis | 2 | 0 -------------------+----------+-------------

INSHORE OCEANIC BIRDS

The inshore zone, according to Wynne-Edwards (1935:240), "extends from high-water mark to a maximum of four or five miles out to sea, including islands and reefs within sight of shore." In Micronesia the majority of the Laridae occur in this zone including such residents as Sterna sumatrana, S. anaetheta, Thalasseus bergii, Anoüs stolidus, A. tenuirostris, Gygis alba. These birds, especially S. anaetheta, Thalasseus, and Anoüs, may venture into the offshore zone. Visitors to Micronesia include several terns which probably normally range in the inshore (as well as in offshore) zones, such as Childonias leucopterus and Sterna hirundo. These birds feed to a considerable extent inside the outer reefs surrounding the lagoons, coming to shore frequently in small or large groups. Gygis alba probably spends considerable time on shore; stomachs examined contained fish, crustaceans and insects, indicating that they obtain some of their food ashore.

OFFSHORE AND PELAGIC OCEANIC BIRDS

Wynne-Edwards (1935:241) defines the offshore zone as extending to the continental edge; however, in Micronesia where small islands rise abruptly out of the ocean's depths, there is no useful way to separate the offshore zone from the pelagic zone. Since certain species go farther from the land than others, the two zones may be combined as a single zone extending beyond the sight of land. Birds which frequent this area beyond the inshore zone but may not range extensively at sea include Fregata, Sula, Sterna fuscata, S. hirundo, S. anaetheta, and others. The Herring Gull (Larus argentatus), which has been taken in the northern Marianas, may be classed with this group although it probably ranges widely in the open sea. Birds which spend considerable time at sea and may seldom approach land include Diomedea nigripes, the petrels (Puffinus and Pterodroma), and possibly the tropic birds (Phaëthon).

In numbers of individuals the birds inhabiting the inshore zones are relatively more numerous than those preferring the offshore and pelagic zones, although 12 of the 18 resident kinds of oceanic birds apparently prefer the offshore zone, while only 6 kinds appear to be restricted primarily to the inshore areas.

FAUNAL COMPONENTS

The oceanic birds were probably among the earliest birds to reach the islands of Micronesia. The presence of phosphate deposits on islands (Fais, Angaur), denoting deposition of guano by oceanic birds (possibly boobies, noddies, sooty terns), indicates long time residence by these birds. A person is prone to think that these deposits must have been made by larger concentrations of oceanic birds than are found in these islands today. Whether there were actually more individuals present during the period of deposition of phosphate in the lagoons of these islands is not known, although the elevation of the lagoons (forming the raised islands of Fais and Angaur) with the resulting freshening of the water probably was a great attraction to oceanic birds, especially to those which prefer to drink fresh water. According to Leonard P. Schultz (in litt.), the abundance of fish in the areas about these Pacific islands has been approximately the same since Pleistocene times, so that there was apparently no greater concentration of fish near these islands to attract large populations of fish-eating sea birds. Probably the time element is of sufficient magnitude to account for such deposition by birds with a population similar to that found there today.

The oceanic avifauna of Micronesia contains birds which are apparently from ancestral homes in the Palearctic Region, in the North and Central Pacific, in Polynesia, in Melanesia and Malaysia, and from homes the positions of which are uncertain because of the widespread circumtropical occurrence of the birds. There are no sea birds that are endemic in Micronesia.

Oceanic birds whose range is in the Northern Hemisphere (especially Palearctica) reach the northern and western edges of Micronesia as winter visitors. These include Larus argentatus, Chlidonias leucopterus, and Sterna hirundo. Another northern gull, Larus ridibundus, has been reported in the Marianas.

One bird of the North and Northcentral Pacific, Diomedea nigripes, reaches the northern Marianas where it has been taken at Agrihan. It is not unlikely that other birds of the North Pacific reach northern Micronesia as occasional visitors.

Species of oceanic birds which are restricted in their distribution to Polynesia and some adjacent islands and which range to Micronesia, either as visitors or residents, include Puffinus tenuirostris, P. nativitatis, Pterodroma rostrata, P. hypoleuca, Sterna lunata, and Procelsterna cerulea. The islands of the vast Pacific basin offer havens for many kinds of oceanic birds. Apparently there has been considerable speciation among sea birds in Polynesia, especially in its marginal areas. Micronesia has received only a small part of this avifauna.

Two terns, Sterna sumatrana and Thalasseus bergii, have reached Micronesia, either directly or indirectly, each from a dispersion point somewhere in the Melanesian or the Malayan area. These two birds are restricted in their ranges to the western Pacific and the Indian oceans.

Many of the species of oceanic birds found in Micronesia have circumtropical ranges. These include Puffinus pacificus, P. lherminieri, Phaëthon, Sula, Fregata, Sterna anaethetus, S. fuscata, Anoüs stolidus, A. tenuirostris, and Gygis alba. Some of these kinds range along continental shores as well as in island archipelagoes. Others, like Gygis alba, are rarely found along the shores of continents or even at coastal islands.

MIGRATORY SHORE BIRDS

Twenty-eight species of shore birds of the families Charadriidae and Scolopacidae have been recorded from Micronesia, and one other of the family Phalaropodidae apparently occurs in the area, making a total of 29 kinds. From the entire Southwest Pacific, Mayr (1945a:28-47) lists 31 species and subspecies of shore birds and mentions six other species which may occur there. Thus, of a possible 37 kinds of shore birds in this large area (which includes Micronesia), 29 are present in the islands of Micronesia. For purposes of discussion, shore birds are here placed in one of two groups: regular visitors or uncommon visitors. A regular visitor is one which has been recorded in the literature or in unpublished field accounts as being frequently observed in Micronesia in periods of migration. An uncommon visitor is one which has been infrequently observed in Micronesia. Of the 28 kinds of shore birds recorded from Micronesia, 17 are classed as regular visitors and 11 are classed as uncommon visitors.

ORIGINAL HOMES OF THE SHORE BIRDS THAT VISIT MICRONESIA

TABLE 3. BREEDING AND WINTERING GROUNDS OF THE SPECIES OF MIGRATORY SHORE BIRDS IN MICRONESIA

PART A. Location of breeding grounds ==================+========+================+=========+========== CLASS | Number | Circumpolar | Asiatic | American ------------------+--------+----------------+---------+---------- Regular visitors | 17 | 5 | 10 | 2 Uncommon visitors | 11 | 2 | 8 | 1 +--------+----------------+---------+---------- Totals | 28 | 7 | 18 | 3 ------------------+--------+----------------+---------+----------

PART B. Location of wintering grounds ------------------+--------+---------+---------+----------+--------- CLASS | Number | Circum- | Asiatic | American | Oceanic |tropical| | | | ------------------+--------+---------+---------+----------+--------- Regular visitors | 17 | 2 | 13 | 1 | 1 Uncommon visitors | 11 | 1 | 8 | 2 | 0 +--------+---------+---------+----------+--------- Totals | 28 | 3 | 21 | 3 | 1 ------------------+--------+---------+---------+----------+---------

Denotes birds which breed on both American and Asiatic sides of the Pacific Ocean.

The shore birds which are known to visit Micronesia breed in the Northern Hemisphere. Table 3 summarizes the data concerning the breeding and wintering areas of these birds. As shown in part A of table 3, 18 of the 28 species which visit Micronesia come from Asiatic breeding grounds. Seven have circumpolar breeding ranges and three (two are regular visitors) come from American breeding grounds. As shown in part B of table 3, 21 of the 28 waders have their winter ranges on the Asiatic side of the Pacific with eastern extensions to Micronesia and other parts of Oceania. Of the 7 remaining species, the winter ranges of three are circumtropical; the winter range of a fourth is restricted to Oceania; and the winter ranges of the remaining three (two classed as uncommon visitors) are American.

Bryan and Greenway (1944:109-115) record 14 species of shore birds from the Hawaiian Islands. One of these, Himantopus himantopus knudsoni, is a resident, probably of New World origin, according to Mayr (1943:56). The others, listed in table 7, include three species unknown in Micronesia. One of these, Phalaropus fulicarus, apparently winters at sea off the west coast of South America. The other two species (Charadrius vociferus vociferus and Gallinago delicata) are classified by Bryan and Greenway as "accidental" and "occasional" visitors from North America. The ten species common to both the Hawaiian Islands and Micronesia include seven whose breeding grounds are circumpolar, two whose breeding grounds are in Arctic America and one whose breeding ground is in Arctic Asia. The winter ranges of these ten species include four which are circumtropical, three which are Asiatic, one which is restricted to Oceania, and only two which are American.

The ability of the shore birds to migrate almost as well over water as over land may explain their spread into Oceania. The likelihood that shore birds, when migrating may have ventured to Micronesia and Polynesia initially from the Asiatic side of the Pacific is strongly suggested by the data given in the paragraph above. Also, on the Asiatic side of the Pacific there are large numbers of islands, which form several archipelagoes extending from Kamchatka south to Malaysia. Once accustomed to migrating along these chains of islands from the Arctic to Australia, birds would probably have to make only minor adjustments to extend the breadth of their migratory routes eastward into the islands of the Pacific Ocean. In contrast, on the Pacific coast of North America there are few coastal or offshore islands and there is a vast area of open water separating the Hawaiian Islands from the American mainland. Probably the vastness of this area of water offers little stimulus to birds to expand their migratory ranges westward, and in part accounts for the small North American contingent in the population of shore birds of the Central Pacific. Some North American shore birds do visit the Pacific. The brisk trade winds from the northeast might be an aid to the birds in their flights from Nearctica to Hawaii.

The long flight now made by shore birds going from the Aleutians to the Hawaiian Islands may have commenced as a gradual expansion from the west, or perhaps such a route was initiated by birds flying northward through the Hawaiian Chain to the Arctic in migrating to their breeding grounds, and then later returning via the same route to reach their wintering grounds.

ROUTES OF MIGRATION

The small and isolated islands of Oceania might, upon first inspection, seem to offer but little attraction to shore birds. Hesse, Allee and Schmidt (1937:172, 173) point out that the "open southeastern Pacific" being least supplied with water from land sources, which is an important means of fertility, is known to have one of the poorest faunas found anywhere in the oceans. However, there are extensive tidal flats, especially on the leeward sides of the islands, and these flats apparently afford extensive feeding grounds for these birds. Also, the absence of competition from resident birds as well as the virtual absence of predatory animals (native man and his domesticated animals excepted) are other factors which may help to make the islands attractive wintering grounds for shore birds.