Routes of Migration

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

	Palaus	Western Carolines	Marianas	Central Carolines	Eastern Carolines	Marshalls
Number of species	20	12	17	11	10	10

Table 5. Shore Birds Which May Use the Asiatic-Palauan Flyway

Regular Visitors		Uncommon? Visitors
Pluvialis dominica fulva		Charadrius dubius curonicus
Charadrius mongolus stegmanni		Charadrius alexandrinus
Charadrius leschenaultii		Calidris tenuirostris
Numenius phaeopus variegatus		Erolia ferruginea
Numenius madagascariensis		Erolia subminuta
Limosa lapponica baueri		Limicola falcinellus sibirica
Tringa nebularia
Tringa glareola
Actitis hypoleucos
Heteroscelus brevipes
Arenaria i. interpres
Gallinago megala
Erolia minuta ruficollis
Erolia acuminata

Table 6. Shore Birds Which May Use the Japanese-Marianan Flyway

Regular Visitors		Uncommon? Visitors
Pluvialis dominica fulva		Squatarola squatarola
Charadrius mongolus stegmanni		Numenius tahitiensis
Numenius phaeopus variegatus		Numenius madagascariensis
Limosa lapponica baueri		Tringa glareola
Actitis hypoleucos		Gallinago gallinago gallinago
Heteroscelus brevipes		Erolia minuta ruficollis
Heteroscelus incanus
Arenaria i. interpres
Gallinago megala
Crocethia alba
Erolia acuminata

Table 7. Shore Birds Which May Use the Nearctic-Hawaiian Flyway

Regular Visitors		Uncommon? Visitors
Pluvialis dominica fulva^*		Squatarola squatarola^*
Numenius tahitiensis^*		Charadrius hiaticula semipalmatus^[+]
Heteroscelus incanus^*		Charadrius v. vociferus
Arenaria i. interpres^*		Limosa lapponica baueri^*
Crocethia alba^*		Tringa melanoleuca^*[+]
Phalaropus fulicarius		Gallinago delicata
Phalaropus lobatus^*?		Erolia melanotos^*
		Erolia acuminata^*

^* Indicates species which are found in Micronesia.

^[+] Indicates species not recorded from the Hawaiian Islands; see Bryan and Greenway (1944:109-115).

Populations of Shore Birds in Micronesia

Although shore birds have been observed in Micronesia on many occasions, actual counts of numbers of individuals of the different birds have rarely been made. Kubary, Finsch, Marche, Seale and other early collectors and observers record some data of this kind as have the Japanese investigators in later times. William Coultas of the Whitney South Sea Expedition obtained considerable information of this nature at Guam, Saipan, Kusiae, Ponapé, and the Palaus, but it is unpublished. His records were made in fall, winter and spring, when migrants were present in large numbers and these observations offer evidence that many of the migrants are comparatively numerous, especially in the Carolines, throughout the winter months. McElroy's observations made on his trip for NAMRU2 to Truk in December, 1945, offer further evidence of this.

Table 8. Populations of Migratory Shore Birds Seen at Guam in 1945

	Chara- drius mongolus	Pluvialis dominica	Numenius phaeopus	Actitis hypo- leucos	Hetero- scelus spp.	Hetero- scelus incanus^[+]	Hetero- scelus brevipes^[+]	Limosa lapponica	Arenaria interpres	Uniden- tified	Total No. of indivi- duals	Total No. of species
March 11	x										x	1
March 17	10		1		2						13	3
March 19	x		x		x				x		x	5
April 24	x										x	1
April 26								1			1	1
May 19					3	2					5	1
May 21					4						4	1
May 26					x	2					x	1
June 1			1								1	1
June 6^*		1	x		x		1			x	x	4
June 11			1								1	1
June 12			12		2						14	2
June 22			2							1	3	2
June 30										2	2	1
July 7			2								2	1
July 8	3		x		1						x	3
July 16^*	6		3	3	4		1				17	4
July 19	x		x		x						x	3
July 24^*	10		6		3		2		3	5	29	5
July 26			8								8	1
August 2			x							x	x	2
August 3			1								1	1
August 6^*			6		12						18	2
September 29	x		x						x		x	2
October 3^*	x				x						x	2
October 10	x		x		x	2			x		x	4
October 11									2		2	1
October 23^*	x		x		x	1	1		x		x	5
October 24	x										x	1

^* Observations made on beach at Agfayan Bay area.

x Observed but numbers not recorded.

^[+] Figures based on identified skins.

None of the above workers, however, obtained very much information on comparative numbers of species.

Tables [8], [9], and [10] present the writer's findings on populations of migratory shore birds in Micronesia in 1945. At Guam, as shown in [table 8], the records for March, April and early May are few, owing to a limited amount of field observation. Beginning in late May and until October 24 a greater amount of time was spent in the field and more regular records were obtained. No observations were made by the author at Guam in the period from August 11 to September 25. The dates marked with an asterisk are those on which observations were made on the extensive tidal flats at Agfayan Bay and vicinity. These flats, at low tide, present excellent feeding grounds for waders and in 1945 were undisturbed by parties of service personnel, because the area was "off-limits."

[Table 8] shows that Pluvialis dominica, Numenius phaeopus, and Heteroscelus spp. were the shore birds most frequently found at Guam in this period. Pluvialis dominica was the most numerous of the three species. Of Heteroscelus there was approximately equal representation of H. incanus and H. brevipes as indicated by specimens collected. These birds were not identified to species in the field.

Although records were made only infrequently in the spring migration, such information as was obtained indicates that the populations were largest in March and early April. On April 24, Pluvialis dominica was the only bird observed on beaches and in upland openings. On April 26, a single Limosa lapponica was recorded. On May 15, no shore bird was seen on a trip along several beaches. In late May and early June, single individuals of Heteroscelus were found. Of this genus, those collected in May were in nuptial plumage, and those collected in June were in winter plumage and probably should be classed as non-migrants. Numenius phaeopus was occasionally recorded beginning in early June, but waders were totally absent from beaches at Agfayan Bay and vicinity on June 18 and 19. Few shore birds were seen in early August. In late September, birds, especially Pluvialis dominica, Numenius phaeopus, and Heteroscelus spp., were numerous. These species were numerous until October 24, when observations were discontinued.

Of the 17 species of migratory shore birds recorded from the Mariana Islands, eight were identified. Of these eight, three species, Limosa lapponica, Actitis hypoleucos, and Charadrius mongolus, were found on only one occasion. Never more than four species were identified on a single field trip. These data give an idea of the lack of variety of species that may be observed on Micronesian islands.

Table 9. Populations of Migratory Shore Birds Seen at Ulithi Atoll in 1945

Species	Island and Date
	Potangeras				Fas- sari	Mange- jang	Pau	Losiep
	Aug. 14	Aug. 15	Aug. 16	Aug. 17	Aug. 19	Aug. 20	Aug. 21	Aug. 22
Pluvialis dominica			6	5	4		10	5
Charadrius mongolus							x	2
Numenius phaeopus	1	4		1			1	2
Actitus hypoleucos							2	2
Heteroscelus spp.					2		6	3
H. incanus^*						1		2
Crocethia alba							30	5
Total No. of Individuals	1	4	6	6	6	1	49	21
Total No. of Species	1	1	1	2	2	1	6	6

^* Figures based on identified skins.

x Observed but numbers not recorded.

[Table 9] lists the shore birds seen at Ulithi Atoll, Caroline Islands, on eight field excursions in the period from August 14 to August 22, 1945. Of seven species of shore birds known to visit the atoll, six were taken in this period. As observed at Guam, Pluvialis dominica and Numenius phaeopus were the species most frequently found. Heteroscelus was seen on three occasions; those collected were identified as H. incanus. Most of the shore birds were seen at Pau and Losiep, islands unoccupied by man. Similar tidal flats are present at most of the other small islands in the atoll, but these islands (Asor, Fallalop, Potangeras, Fassarai and Mangejang were visited) were occupied by small detachments of service personnel or by natives, which may have tended to keep many of the shore birds away. At the more populated islands of Asor and Fallalop, no shore birds were seen. Almost as many species were recorded at Ulithi on the eight field trips as were found by the author at Guam in eight months of observations.

Table 10. Populations of Migratory Shore Birds Seen at the Palau Islands in 194

Species	Peleliu								Angaur
	August		September						Sept. 21
	24	28	1	6^*	8^*	9^[+]	16^*	20^*	Sept. 21
Pluvialis dominicaa	x		x	x	25		20	x	x
Charadrius mongolu				x	25		5	x	x
C. leschenaultii				x	25		5	x	x
Numenius phaeopus			3	x	30		20	x	x
N. madagascariensis					1	1		15
Limosa lapponica					3		4
Tringa nebularia		6					3
T. glareola^[++]									1
Actitis hypoleucos						2
Heteroscelus sp.			x	x	75		x	x	x
H. brevipes^[++]				3	2		2
Arenaria interpres					20
Capella megala									4
Calidris tenuirostris					15		20
Erolia minuta				x	50		50	x	x
E. acuminata^[++]									3
E. ferruginea^[++]				1
Limicola falcinellus^[++]									1
Unidentified	x			x	x		x	x	x
Total No. of Individuals	x	6	x	x	271+	3	129+	x	x
Total No. of Species	1	1	3	7	10	2	9	7	10

^* Observations made on beaches at Akarakoro Point, Peleliu.

x Observed but numbers not recorded.

^[+] Observations made at fresh water ponds.

^[++] Figures based on identified skins.

[Table 10] presents field counts at the Palau Islands in the period from August 24 to September 21, 1945. Of 20 species of shore birds known from the Palaus, 17 species were collected or identified on this trip. It was apparent that the fall migration was at its height at this time. Birds were numerous at inland openings and ponds, air field strips, and on the extensive tidal flats at Akarakoro Point. The latter area is between Peleliu and the adjacent island of Ngesebus to the north. Several observations were made at this area (as indicated by the dates marked with asterisks in the table); on September 8, 271+ shore birds were counted; on September 16, 129+ were counted. Six species were observed to be abundant. The majority of the birds found at these beaches were in small flocks which consisted of several birds of one or more species.

The birds observed at Angaur on September 21 were seen at several fresh and brackish ponds. Four species (Tringa glareola, Erolia acuminata, Limicola falcinellus, Gallinago megala), which were not taken on the tidal flats or elsewhere at Peleliu, were found at these ponds.

The abundance, and more especially the variety, of shore birds at the Palau Islands during this period was in marked contrast to the smaller and less diversified populations of shore birds in rather similar insular environments at Ulithi and Guam. These differences offer support for the supposition that the Asiatic-Palauan Migratory Shore Bird Flyway is distinct from the Japanese-Marianan Migratory Shore Bird Flyway.