Interactions between common buzzard Buteo buteo and goshawk Accipiter gentilis : trade‐offs revealed by a field experiment

I examined the behavioural interactions between common buzzard Buteo buteo and goshawk Accipiter gentilis and their effects on buzzard breeding success and brood defence with a two‐year field experiment using dummies and playback calls. A priori I showed through an extensive nest site analysis that there is considerable nesting habitat overlap between the two species and hence potential for interspecific competition for prime nesting habitat. Buzzards had a significantly lower breeding success when presented with a goshawk dummy compared to control broods but there was no effect of buzzard dummies on reproductive success. Buzzards failing with their breeding attempt tended to select another nest site while successful buzzards more frequently used the same nest again. Buzzard pairs were less often attacked by common crows Corvus corone while exposed to goshawk dummies compared to buzzard dummies. The decision to desert a nest seems to be a trade‐off between predation risk on the one hand and protection against crows on the other. Goshawks proved far more aggressive against an intraspecific dummy than buzzards. Buzzards adjusted their level of brood defence against both intra‐ and interspecific dummies according to the age of offspring but not offspring number, with an increasing brood defence level with increasing offspring age. Thus the behaviour of buzzards towards goshawks is a result of a complex system of trade‐offs between predation risk, competition for prime nesting habitat and protection from crows on which brood value acts as a temporal modifier.