The second major migration system for Palaearctic nesting birds, the Asian-Australasian system, has been less studied, but provides interesting comparisons (Lane & Parish 1991). Approximately 234 species are involved, derived from a breeding area extending eastward from central Asia across to Alaska. These species winter in the southeastern part of the Eurasian land mass, including the Indian subcontinent (about 4 million km2) and the southeast Asian mainland (about 2 million km2), but some species extend southwards to Australia and New Zealand and others eastward to some Pacific Islands. The natural vegetation of Southeast Asia, where most of the migrants spend the northern winter, is rainforest or dry deciduous woodland, with some savannah and grassland. But much of this natural habitat has disappeared under human impact, becoming agricultural or arid through deforestation, scrub removal and overgrazing. In general, Southeast Asia is much wetter than Africa but, as in Africa, those migrants that stay north of the equator experience progressively drier conditions during their stay. Vegetation dies back and flood-pools dry. Tropical wintering areas that lie south of the equator, including New Guinea and northern Australia, are at their wettest in the northern winter (austral summer), but temperate southern Australia and New Zealand have their driest weather then.
The further south that birds migrate within Asia, the more mountains they face, the more water gaps they must cross, and the more restricted is the land area that is available to them. Although movement is essentially broad-front, the distribution of land areas tends to channel the birds along particular routes, as confirmed by both observations and ring recoveries. Several island groups provide potential stepping stones, from the Aleutians in the north to the Indonesian and other islands in the south. One major route extends south from Japan and eastern China, through the Philippines; another runs into and through the Malaysian peninsula, while a third runs from south Vietnam southwest across the South China Sea to Borneo (McClure 1974, Medway & Wells 1976, Ellis et al. 1994). Some birds also cross the Gulf of Thailand to and from the Malaysian peninsula.
In general, as in Africa, most migrant species stay in the northern part of this wintering area, and species numbers decline with increasing distance southwards to New Zealand. About 161 migratory species filter into Malaysia, but most stop in Indonesia, at about 10°N. Few Eurasian passerine species reach New Guinea or Australia, but many shorebirds reach Australia and New Zealand. Numbers of landbirds also decline eastwards out from the Asian mainland onto Pacific
Islands, with no fewer than 120 species wintering regularly in the Malay archipelago, 84 in the Philippines, 38 on Celebes, 10 in the New Guinea area and three in the Bismarck Archipelago (Mayr 1969). Yet other shorebird species from Alaska winter on various Pacific islands from Hawaii across to Indonesia, and southward to Australia and New Zealand.
The only Asian landbirds that migrate in large numbers to Australia are two species of swifts (Fork-tailed Swift Apus pacificus and White-throated Needletail Hirundapus caudacutus), while others that reach Australia in smaller numbers include the Oriental Cuckoo Cuculus saturatus, the Barn Swallow Hirundo rustica gutturalis, Red-rumped Swallow H. daurica, Grey Wagtail Motacilla cinerea, Yellow Wagtail M. flava and the Oriental Reed Warbler Acrocephalus orientalis (Dingle 2004). At least one of these, the Oriental Cuckoo, also reaches New Zealand. The scarcity of Palaearctic songbird migrants in Australia during the northern winter contrasts with their abundance in southern Africa.
On the other hand, about 33 species of arctic-nesting shorebirds winter in Australia, totalling some 2-3 million individuals, nearly half the total flyway population (Lane & Parish 1991). The most abundant coastal species include the Bar-tailed Godwit Limosa lapponica (165 000), Little Curlew Numenius minutus (180 000), Great Knot Calidris tenuirostris (270 000), Red Knot Calidris canutus (153 000), Red-necked Stint Calidris ruficollis (353 000) and Curlew Sandpiper Calidris ferruginea (188 000) (Higgins & Davies 1996). These birds mostly arrive in the north of Australia in late August and September. Some species, such as Greater Sand Plover Charadrius leschenaultii and Great Knot, mostly remain in the north of Australia through the non-breeding season, while others, such as Rednecked Stint and Curlew Sandpiper, mostly move on to coastal sites further south. Some wader species winter inland, such as the Oriental Pratincole Glareola maldivarum, Common Sandpiper Actitis hypoleucos and Greenshank Tringa nebularia, all of which move to the coast in extreme drought years. The Bar-tailed Godwit and Red Knot reach New Zealand in large numbers (57 000 and 36 000 respectively), as do smaller numbers of the other species (Sagar 1986, Higgins & Davies 1996). Further shorebird species from North America also winter in Australia and New Zealand. Among other waterbirds, the Garganey Anas querquedula and White-winged Black Tern Chlidonias leucopterus reach Australia, as do several marine tern and skua species. The main threat to these migratory birds, besides coastal drainage, is probably human hunting, with an estimated 1.5 million shorebirds killed each year in Indonesia alone.
It is not hard to imagine why so few Palaearctic species reach Australasia. This land area lies at the end of the migration route, and also well to the east of the Eurasian land mass. Secondly, Australia offers relatively small areas of habitat suitable for most Eurasian landbird migrants; and thirdly, several water crossings are necessary to reach these areas, by far the longest being the 1600-2000 km stretch between Australia and New Zealand.
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