Abstract

Aerial surveys between 1981 and 1984 were used to identify monthly trends in the abundance of Australian pelicans (Pelecanus conspicillatus), darters (Anhinga melanogaster), little pied cormorants (Phalacrocorax melanoleucos), and whiskered terns (Chlidonias hybrida) on five floodplains of the Alligator Rivers region, 250 km east of Darwin in the monsoonal Northern Territory. Ground surveys were conducted during the same period on one of the floodplains, the Magela plain. The aerial surveys indicated that the Magela floodplain was inhabited by few of these birds during the wet season (November-March), but that numbers then increased substantially in the dry season. The Nourlangie floodplain showed similar patterns, but the numbers of birds tended to be lower. Birds were generally uncommon on the shallower East Alligator and Cooper floodplains. Ground surveys suggested that the birds sought out the persistent swamps that characterise the Magela floodplain in the dry season. Ground surveys also indicated that aerial surveys underestimated densities; on the basis of correction factors calculated from ground surveys, peak numbers on the five floodplains were roughly estimated to be about 2000 darters, 9000 little pied cormorants, 55 000 Australian pelicans and 50 000 whiskered terns. Little black cormorants (Phalacrocorax sulcirostrus) were sometimes abundant, but their sporadic occurrence prevented analysis of seasonal trends. Australasian grebes (Tachybaptus novaehollandiae), great cormorants (Phalacrocorax carbo), pied cormorants (Phalacrocorax varius), buff-banded rails (Rallus philippensis), Baillon's crakes (Porzana pusilla), white-browed crakes (Poliolimnas cinereus), purple swamphens (Porphyrio porphyrio), Eurasian coots (Fulica atra), silver gulls (Larus novaehollandiae), white-winged terns (Chlidonias leucoptera), gull-billed terns (Gelochelidon nilotica) and Caspian terns (Hydroprogne caspia) were recorded in low numbers. The Alligator Rivers region acted as an important dry season refuge because of the unusually persistent fresh waters of the Magela and Nourlangie floodplains and some of the backswamps of the South Alligator, such as Boggy Plain.