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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.

DIetary Shifts Based Upon Prey Availability in Peregrine Falcons and Australian Hobbies Breeding Near Canberra, Australia

ABSTRACT We collected prey remains and pellets at 16 Peregrine Falcon (Falco peregrinus) nest territories (975 prey items from 152 collections) and one Australian Hobby (F. longipennis) territory (181 prey items from 39 collections) during four breeding seasons in two time periods: 1991–1992 and 2002–2003, a total of 60 peregrine nest-years and three hobby nest-years. By number, European Starlings (Sturnus vulgaris) were the main prey taken by both falcons in 1991–1992 and 2002–2003, but starlings made up a smaller percentage of the diet by number in the latter period, apparently because their numbers had declined in the wild. Although the geometric mean of prey weights and geometric mean species weights were similar in the two time periods, both falcons compensated for the decline in European Starlings in the latter period by taking a greater variety of bird species, particularly small numbers of mostly native birds, rather than taking more of one or two other major prey species. Peregrines took 37 bird species in the latter period not found among their prey remains in the earlier period, and more individuals of some large species such as Gang-gang Cockatoos (Callocephalon fimbriatum), Galahs (Cacatua roseicapilla), and Rock Pigeons (Columba livia). Prolonged drought and competition from increasing numbers of Common Mynas (Acridotheres tristis) may have caused European Starlings to decline, but neither falcon species compensated by catching Common Mynas, even though they are of optimal prey size. Though both falcons, especially peregrines, took the more common bird species found in surveys in the Australian Capital Territory, both tended to avoid large or dangerous species, or agile species that foraged close to cover, such as Common Mynas.