The Effect of Food Stealing by Silver Gulls Larus novaehollandiae on the Foraging Efficiency of Bar-tailed Godwits Limosa lapponica

Abstract

Silver Gulls Larus novaehollandiae stealing food from Bar-tailed Godwits Limosa lapponica were studied at three intertidal sand and mudflat sites on the estuary of the Parramatta River, New South Wales, between December 1993 and March 1994. Godwits preyed exclusively on polychaete worms ranging from 2-9 cm in length. Densities of foraging gulls were similar at the three sites, at 30.0, 26.4 and 27.4 birds/ha. but godwit densities were dissimilar, at 26.4, 2.9 and 1.0 birds/ha. Frequencies of prey stealing attempts by gulls were 0.7, 0.23 and 0.70 per 10 min of godwit foraging time and were not related to the relative densities of gulls and godwits, nor to differences in the rates at which godwits caught prey. The godwits lost 0.6%, 0.1% and 1.0% of their prey to tht gulls and 3.0%, 0.2% and 2.5% of their foraging time through increased vigilance and avoidance of gulls. It was concluded that the gulls did not have a significant effect on the foraging efficiency of the godwits.