Trophic relationships between neighbouring White-bellied Sea-Eagles (Haliaeetus leucogaster) and Wedge-tailed Eagles (Aquila audax) breeding on rivers and dams near Canberra

Abstract

The diet of the White-bellied Sea-Eagle (Haliaeetus leucogaster) in Australia is poorly known, especially inland. The diet of the Wedge-tailed Eagle (Aquila audax) is better known, but the overlap in prey used by the two eagles has not been studied. In four inland territories of White-bellied Sea-Eagles, and five territories of Wedge-tailed Eagles nesting close to them (range 1.6-5.1 km apart) between July 2002 and December 2004, we identified 116 and 118 prey items from nests of White-bellied Sea-Eagles and Wedge-tailed Eagles respectively. There was little overlap between the diets, and that of Wedge-tailed Eagles was similar to that reported elsewhere. In addition to fish, White- bellied Sea-Eagles specialised in aquatic birds, such as cormorants, grebes or ducks, and aquatic reptiles, such as turtles or water dragons, but tended to avoid terrestrial birds and reptiles, such as ravens and skinks, or mammals such as European Rabbits (Oryctolagus cuniculus) and macropods that were the main prey of nearby Wedge-tailed Eagles. Though food niche breadth was almost identical for the two eagles, Wedge-tailed Eagles captured significantly larger prey, as indicated by the geometric mean prey weight. Our results indicate that closely located breeding pairs of riparian Wedge-tailed Eagles and White-bellied Sea-Eagles were not competing for food, owing to the differences in foraging preferences, at least during the breeding season. We found no evidence to support the claim that the spread of rabbits assisted the increase of breeding numbers of White-bellied Sea-Eagles.