Assessing the diet of birds of prey: a comparison of prey items found in nests and images

Abstract

Prey remains and regurgitated pellets collected from nests are the most common material for investigating the diet of birds of prey. Generally such data is thought to be biased with large prey overrepresented. However, there is no analysis investigating how systematic the error is in relation to prey size, abundance, species or method used. In this study we compared the diet composition of the Goshawk (Accipiter gentilis) and the Buzzards (the Common Buzzard Buteo buteo and the Rough-legged Buzzard B. lagopus) in northern Finland obtained indirectly (by collection of prey remains and pellets) and by direct methods (using a movie camera and a video recording system). In order to investigate the relationship between these two types of diet data more generally, we combined our own material and some published original data. Video and film images allowed us to identify according to class or family level most of the prey items delivered to the nests during the surveillance sessions, but identification according to genus or species level often was difficult. We found that small prey items were underestimated in remains as compared to large prey items. However, when none of the prey delivered to the nest is in large numbers, prey remains give fairly reliable idea of the real diet.