Radar And Visual Observations Of Autumnal (Southward) Shorebird Migration On Guam

-Several species of shorebirds migrate between eastern Asia and the southern Pacific islands, Australia, and New Zealand. Observations made from Guam (13?25'N, 144?45'E) during autumn 1983 indicate that a significant number of birds take a direct route over the western Pacific Ocean. Radar observations and ground counts of migrants on Guam showed two periods of autumnal migratory activity. The first, largely adult birds, was in August and September. The second, largely juveniles, was in late September and October. Radar indicated that large numbers of birds passed over the island to the south with no evidence of compensation for drift by the easterly winds. Comparison of radar and ground observations on Guam showed that only a small subset of migrants stop on the island, suggesting that some species may make nonstop flights between eastern Asia and the South Pacific. Received 3 August 1987, accepted 22 February 1988. ORNITHOLOGISTS in Europe and North America can draw on more than a century of migration studies using bird banding and on 40 years of radar observations. The study of migration patterns from Asia to Australia and the South Pacific is comparatively recent. McClure (1974) reviewed autumn banding records of several species of shorebirds (Charadriiformes) from southeast Asia, the Philippines, eastern China, and Japan. The movement was primarily southwest along the mainland coast to southeast Asia and the Philippines, with a few recoveries as far south as Australia. Weishu and Purchase (1983) reviewed the preliminary results of the Australian Bird Banding Scheme and reported several recoveries of northbound shorebird migrants to China from Australia but no recoveries of southbound birds returning to Australia. These data suggest a southward movement over open water rather than along a coastal route. Reviews of the distribution of shorebirds indicate that several species migrate between eastern Asia and Micronesia (Baker 1951; Owen 1977; Pyle and Engbring 1985, 1987), Australia (Lane 1987), and New Zealand (Davies 1986, Sagar 1986). In the western Pacific islands the primary migrant shorebirds are Lesser Golden-Plovers (Pluvialis dominica fulva) and Ruddy Turnstones (Arenaria interpres) (Bell 1948; Johnson 1973, 1979; Smart 1973; Thompson 1973; Jenkins 1981; Schipper 1985; Pyle and Engbring 1985; Williams and Grout 1985; Hayes 1986). Nonshorebird migrants to Micronesia are rare, with the exception of the Cattle Egret (Bubulcus ibis) (Pyle and Engbring 1985, 1987; Williams and Grout 1985). We conclude from these studies that significant numbers of Lesser Golden-Plovers and Ruddy Turnstones, as well as smaller numbers of other shorebirds, including both tattler species (Heteroscelus incanus and H. brevipes), migrate annually from northern breeding grounds to small islands and atolls in the western and southern Pacific. Bar-tailed Godwits (Limosa lapponica) and Red Knots (Calidris canutus) also make regular migrations in large numbers from breeding grounds in northern and eastern Asia to New Zealand. These species and the Great Knot (C. tenuirostris), Rufous-necked Stint (C. ruficollis), Sharp-tailed Sandpiper (C. acuminata), and Curlew Sandpiper (C. ferruginea) make regular annual migrations to Australia. There is insufficient evidence at present to determine whether their flights are primarily over southeast Asia or whether they take a more direct route over the western Pacific. Migrants not regularly seen in Micronesia may make a long, nonstop flight passing over the islands, as suggested for the Hudsonian Godwit (Limosa haemastica) in the Atlantic (Hager 1966). The Great Knot is large enough to fly nonstop between the north coast of Australia and Shanghai, a distance of 5,500 km (Barter 1986). Southward migratory activity of shorebirds in Micronesia commences in early to mid-August and continues through October, with two peak periods of arrivals. This migratory pattern 460 The Auk 105: 460-466. July 1988 This content downloaded from 207.46.13.52 on Fri, 21 Oct 2016 05:50:54 UTC All use subject to http://about.jstor.org/terms July 1988] Shorebird Migration over Guam 461 is similar to that on Hawaii, where most adult Lesser Golden-Plovers arrive in late August and either remain as residents during the nonbreeding ("winter") season or resume migration to more southerly islands throughout the Pacific. Juveniles arrive in Hawaii in late September and October (Johnson et al. 1981, Johnson and Johnson 1983). A similar 4-6 week time lapse occurs between adult and juvenile Ruddy Turnstones passing through the Pribilof Islands on their southward migration (Thompson 1973). Southward movements over the western Pacific are comparable to southward migrations over the western North Atlantic Ocean (Williams et al. 1977, Richardson 1980, Williams 1985). This nonstop overwater flight significantly shortens the route from eastern North America to South America and makes use of prevailing wind patterns to assist the flight. If the patterns of migration in the western Atlantic are determined primarily by geography and wind patterns, we would predict similar migration routes for the western Pacific where a direct overwater crossing offers similar advantages. Guam, located in the Mariana Islands 130 north of the equator and about 3,000 km from the Asian mainland, offered a location well suited to test this prediction. Comparisons of radar observations with counts of birds on the ground at continental sites often report poor agreement on the density of migration (Eastwood 1967, Williams et al. 1981). The poor correlation might be due either to groups of birds flying over large areas of a continent without landing or to the observers' inability to survey systematically a significant part of a continental area. Such comparisons gave good agreement at Maui, Hawaii, where a significant proportion of the island habitat could be surveyed and the island was a terminus of migration (Kloeckner et al. 1982). Observations on Guam offered an opportunity to test whether the correlations on Maui were due to the better survey conditions on isolated islands or to the fact that most migrants in the area landed on the island. MATERIALS AND METHODS Visual observations were made daily from 16 August through 22 October 1983 at Dungcas Beach on Guam, Mariana Islands (13?25'N, 144?45'E) as close as possible to 23 cm above mean low water on a rising tide (Fig. 1). Two observers counted shorebirds on the D