. Bulletin of the British Ornithologists' Club. Birds. Richard Schodde 216 Bull. 1991 111(4) NORTH WEST AUSTRALIA ^, Darwin/. Figure 1. Localities for records of Asian Gull-billed Terns in Australia. or immature plumage. Otherwise they match Asian affinis in their dis- tinctly steel-grey dorsum and upper tails, small size, fine bills and short tarsi (cf. Roselaar in Cramp & Simmons 1975: 16-17). Based on 11 adult males and 10 females examined from Australian museums, macrotarsa is much paler whitish-grey over the back and wing coverts, almost snow- white on the tail, larger (flatten
. Bulletin of the British Ornithologists' Club. Birds. Richard Schodde 216 Bull. 1991 111(4) NORTH WEST AUSTRALIA ^, Darwin/. Figure 1. Localities for records of Asian Gull-billed Terns in Australia. or immature plumage. Otherwise they match Asian affinis in their dis- tinctly steel-grey dorsum and upper tails, small size, fine bills and short tarsi (cf. Roselaar in Cramp & Simmons 1975: 16-17). Based on 11 adult males and 10 females examined from Australian museums, macrotarsa is much paler whitish-grey over the back and wing coverts, almost snow- white on the tail, larger (flattened wing £S 304-344, $? 302-348 mm), heavier in bill (exposed culmen £S 42-47, 9$ 39-43 mm, depth of bills at feather line SS , 9$ mm), and longer in foot (tarsus $$ 34—38, 99 32-37 mm); cf. measurements of affinis and nominotypical nilotica given by Roselaar (loc. cit.). These records and those of McKean (1981) lend support to the suggestion of Johnstone (1977) that affinis reaches the tidal waters and estuaries of northwestern Australia frequently during the austral summer. During a CSIRO faunal survey of the Alligator Rivers region between 1971 and 1974, small, white-crowned Gull-billed Terns match- ing affinis were recorded commonly between October and April, suggest- ing that it is a regular wintering migrant. The terns were solitary, quartering the saline or brackish lower estuaries of the Alligator Rivers up to 15 km inland and diving from great heights (often 100 m) to take small fish and Crustacea. In contrast, endemic Australian macrotarsa also reaches the region on non-breeding dispersal but mainly though not exclusively in the intervening period, May—November {cf. McKean 1981). Around the Alligator Rivers it was found foraging only over fresh- water swamps, billabongs and flooded plains further inland to take aquatic insects as much as fish and Crustacea. Data on diet were gathered from the stomachs of collected specimens (2 affinis
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