Assessing prey provisioned to Common Buzzard Buteo buteo chicks: a comparison of methods

Capsule: Methods of assessing raptor diet carry significant inherent biases which can vary over time.Aims: To compare methods of assessing Common Buzzard Buteo buteo diet composition and assess how any differences vary between years.Methods: Diet was assessed at 32 Common Buzzard nests on an area of upland heather moorland in Britain, managed for Red Grouse Lagopus lagopus scotica, over three years. Data obtained from nest cameras were compared with data from prey remains and regurgitated pellets.Results: Diet composition differed between methods in all years. Methodological differences varied between years in relation to an almost twelve-fold change in Field Vole abundance, a key prey of Common Buzzards, while abundances of alternative prey changed little. Small mammals were underestimated by prey remains in all three years, while herpetofauna were underestimated by prey remains and pellets in two years. Large birds were overestimated by prey remains, significantly so in one year. Pellets overestimated invertebrates in all years. By combining prey remains and pellets, significant yearly variations in biases were eliminated, although the combined measure overestimated large birds and invertebrates.Conclusion: Our results suggest that future studies should consider not only how chosen methods may affect results, but also how effects can differ between years.