[Illustration: FIG. 6. Routes of migration used by shore birds in the
Pacific area. From west to east these are: The Asiatic-Palauan Flyway,
the Japanese-Marianan Flyway, the Nearctic-Hawaiian Flyway.]
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.
Only a few birds have been banded in the Pacific, and the knowledge
which comes from the recovery of banded birds gives but little aid to
the student of movements of birds in the Pacific. The probable flyways
for migratory shore birds there have to be deduced from sight records,
data from specimens collected, known stations of breeding and wintering
(summarized by Peters, 1934:234-293), and from a study of maps of the
region. Analysis of information from the above-mentioned sources
indicates that there are three routes taken by shore birds which migrate
from Micronesia to and from their northern breeding grounds (see figure
6): (1) Asiatic-Palauan Flyway; (2) Japanese-Marianan Flyway; (3)
Nearctic-Hawaiian Flyway.
1. ASIATIC-PALAUAN FLYWAY. For shore birds, there appears to be a
migration route extending almost due south from the Riu Kiu and the
Japanese islands to the Palau Islands. Some birds may migrate _via_ the
Philippines and others may pass to the east of the Philippines. This
route is considered to be distinct from that used by birds which follow
the Asiatic Coast and coastal islands, because the Palau Islands are
situated approximately 600 miles east of the Philippines. Moreover,
there are fewer species--only 20 recorded from the Palaus as compared
with the number recorded from islands closer to the mainland of Asia.
Delacour and Mayr (1946:68-74) list 46 species of shore birds from the
Philippines; the Hand-list of Japanese Birds (Hachisuka _et al_, 1942)
lists 34 species from the Riu Kiu Islands.
The information available indicates that migrant shore birds which
utilize this flyway move east into the Carolines (examples, _Tringa
nebularia_, _Charadrius leschenaultii_); however, the recording of 20
species from the Palaus as compared with only 12 species in the western
Carolines (table 4) indicates that this spread eastward may not be very
pronounced. Migrants in autumn probably move from the Palaus in a
southerly direction toward the New Guinea area. Eight species of shore
birds which reach the Palaus (and adjacent islands in the western
Carolines), are not recorded from other parts of Micronesia. Species
which apparently utilize the Asiatic-Palauan Flyway are listed in table
5.
2. JAPANESE-MARIANAN FLYWAY. Shore birds from Asiatic, and probably
Aleutian and Alaskan, breeding grounds may follow the Asiatic Coast or
the adjacent island chains southeast to the Japanese Archipelago. From
there some of the birds apparently fly south through the Bonin and
Volcano islands to the Marianas, from where they may spread in fanlike
fashion to the southeast, south and southwest, even reaching to the
Palau Islands (example, _Heteroscelus incanus_). The number of species
of shore birds recorded from the Marianas (see table 4) is greater than
that found in the Carolines, but it must be remembered that more
intensive investigations have been made by ornithologists in the
Marianas, which might account for the recording of more species
(especially stragglers, such as _Gallinago gallinago_). Species which
apparently use this flyway are named in table 6.
3. NEARCTIC-HAWAIIAN FLYWAY. Shore birds from breeding grounds in
western Canada, Alaska, the Aleutians, the Bering Sea area, and probably
northeastern Asia may fly in a southerly direction along a broad front
to the Hawaiian Islands. This flyway is probably the one which supplies
to central and eastern Oceania the largest wintering populations of
shore birds. From the Hawaiian Islands birds may fly directly south
through the scattered islands to southern Polynesia, or they may fly in
a southwesterly direction and reach the Marshall Islands. The shore
birds which visit the Marshall Islands apparently move south through the
Gilbert, Ellice and other more southern island groups rather than west
into the Carolines as exemplified by the fact that _Numenius
tahitiensis_, a characteristic migrant through the Marshalls from the
Hawaiian Islands, is rarely found west of the Marshall Islands in
Micronesia. Species which apparently use this flyway are listed in table
7.
Flyways additional to the three suggested above may be utilized by some
shore birds on their southward (and northward) migrations. Species
reaching Wake and the Marcus Islands may fly directly south from the
islands of the North Pacific. Bryan (1903:115, 116) lists four species
of shore birds from Marcus (_Erolia acuminata_, _Heteroscelus incanus_,
_Pluvialis dominica_, _Arenaria interpres_).
TABLE 4. LIST OF SPECIES OF SHORE BIRDS KNOWN FROM FIVE GEOGRAPHICAL
AREAS OF MICRONESIA
Western Central Eastern
Palaus Carolines Marianas Carolines Carolines Marshalls