Interaction between the White-tailed Eagle and Common Buzzard estimated by diet analysis and brood defence behaviour

Abstract

Recent evidence suggests that the White-tailed Eagle (Haliaeetus albicilla) - the largest avian predator to have recently recovered in Europe - may prey on mesopredators and their broods. Mesopredators may respond to predators near their nests by exhibiting offspring defence behaviour. The present study involved White-tailed Eagle diet analysis, and we report on their predation on Common Buzzard (Buteo buteo) nestlings and the results of a field experiment conducted near the nests of Common Buzzards to test how they respond to the appearance of a top predator. The remains of nestling Common Buzzards were observed in 9% of successful nests, while evidence on the consumption of other raptor species was scarce. Additionally, our results indicate that habitat and pair/territory identity may explain the probability of predation on mesopredators, thus suggesting a spatially varying effect on smaller raptors. During the field experiment, Common Buzzards responded to a White-tailed Eagle dummy placed near nest sites during only half of the observed visits. Moreover, the response of Common Buzzards was not explained by their proximity to the breeding pairs of White-tailed Eagles. The probability of Common Buzzards alarm calls and attacks increased in the latest stage of the breeding season, although this was largely due to differences in defence behaviour between pairs, which suggests individual differences in aggressiveness towards predators. In summary, our data indicate that the White-tailed Eagle may prey upon the broods of Common Buzzard under certain environmental conditions, and that this source of prey could easily be accessed due to the weak brood defence behaviour by Common Buzzards.