Measures of predator diet alone may underestimate the collective impact on prey: Common buzzard Buteo buteo consumption of economically important red grouse Lagopus lagopus scotica.

Human-wildlife conflicts often centre on economic loss caused by wildlife. Yet despite being a major issue for land-managers, estimating total prey losses to predation can be difficult. Estimating impacts of protected wildlife on economically important prey can also help management decisions to be evidence-led. The recovery in population and range of common buzzards Buteo buteo in Britain has brought them into conflict with some gamebird interests. However, the magnitude of any impact is poorly understood. We used bioenergetics models that combine measures of buzzard abundance from field surveys with diets assessed by using cameras at nests, prey remains and pellet analysis, to estimate their impact on red grouse Lagopus lagopus scotica on a large (115 km2) moor managed for red grouse shooting in Scotland. Whilst grouse consumption by individual buzzards was lower than previous estimates for other raptor species present on our study site, total consumption could be greater given an estimated 55-73 buzzards were present on the study site year-round. Averaging across diet assessment methods, consumption models estimated that during each of three breeding seasons (April-July 2011-2013), the buzzards foraging on our study site consumed 73-141 adult grouse and 77-185 chicks (depending on year). This represented 5-11% of adult grouse present in April (22-67% of estimated adult mortality) and 2-5% of chicks that hatched (3-9% of estimated chick mortality). During two non-breeding seasons (August-March), consumption models using pellet analysis estimated that buzzards ate a total of 242-400 grouse, equivalent to 7-11% of those present at the start of August and 14-33% of estimated grouse mortality during the non-breeding season. Buzzard consumption of grouse has the potential to lead to non-trivial economic loss to grouse managers, but only if buzzards predated the grouse they ate, and if grouse mortality is additive to other causes.