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Migratory activity of waders departing from north-western Australia in March-April 1991 was recorded by field observations and radar tracking. Field observations showed that the species concerned were mainly Bar-tailed Godwit Limosa lapponica, Grey Plover Pluvialis squatarola and Great Knot Calidris tenuirostris. Peak migration took place in the second week of April. Most flocks departed in the late afternoon, never at high tide. A strong correlation was found between the tidal cycle and the timing of departures. The majority of the flocks flew towards NNW. Given that the next stopover site of especially the larger sized waders is in east and south China (Barter & Wang 1990) this observed departure direction is more westerly than a course along the great circle route (0-degree). Wind patterns along two possible northward routes (a great circle and an island hopping route along the island-arc of South-east Asia) are analysed. In view of recoveries of waders ringed in Australia, especially the smaller waders (Mongolian Plover Charadrius mongolus, Large Sandplover Charadrius leschenaultii, Red-necked Stint Calidris ruficollis, Curlew Sandpiper Calidris ferruginea, Broad-billed Sandpiper Limicola falcinellus, Terek Sandpiper Xenus cinereus) may migrate via the island hopping route, while the larger ones (Grey Plover, Great Knot, Red Knot Calidris canutus, Bar-tailed Godwit, Whimbrel Numenius phaeopus and Eastern Curlew Numenius madagascariensis) take a direct, great circle flight to China. Considerable tail wind assistance can be gained during the northward trip, more so along the island arc than along the great circle. Flight range estimates indicate that the smaller waders cannot fly the 4500-5500 km nonstop to east and south China, while the larger species can, on the condition that they experience tail wind assistance. |
