The breeding of the Wedge‐tailed Eagle Aquila audax in relation to its food supply in arid Western Australia

Abstract

The breeding of the Wedge‐tailed Eagle Aquila audax was investigated in four areas of the arid zone of Western Australia from 1968 to 1976, with some supporting data from the adjacent Mediterranean zone. Breeding was dependent on, and geared to, the occurrence of a minimum level of available prey. In the arid zone the seasonally most regular occurrence of the low rainfall, combined with greatest diversity of vegetation and prey, was associated with the most regular breeding and the greatest annual productivity of young per pair. At the other extreme, where an identical mean annual rainfall was distributed more erratically between seasons and years and was combined with a less diverse flora, the prey available to eagles was restricted to virtually one species–the introduced European rabbit Oryctolagus cuniculus. In this situation there were gaps of up to 4 years between the breeding of eagles, because of major fluctuations in the size of the population of its rabbit prey. Their density was actually greatest in those areas where the widely fluctuating food supply occasionally allowed them to breed with considerable success; however, this did not compensate fully for the irregularity of breeding. Where the prey available was thus restricted to a single species and was least stable, the area may have been unable to support a population which could maintain itself without recruitment from outside.Breeding success in the Mediterranean zone was higher than in the arid zone. The Wedge‐tailed Eagle's breeding strategy is conservative (with ’k‐selected‘ characteristics), but this study shows that it is ecologically successful in both zones, although least so where the climate is extremely erratic.