Exploring the impact of common buzzard Buteo buteo predation on red grouse Lagopus lagopus scotica

Abstract

The relationship between raptors and red grouse Lagopus Lagopus scotica is one of the most topical and contentious wildlife management issues in Britain. The common buzzard Buteo buteo is a generalist raptor which has increased in population and range in Britain during the last 40 years, which in most areas represents a recovery following historical declines. Increasingly, this has reignited conflict with managers of gamebirds concerned about the impact of buzzard predation. Whilst the impact of buzzards on reared pheasants Phasianus colchicus has previously been assessed, the impact of buzzards on red grouse has not been investigated. I aim to address this knowledge gap by providing an insight into the predator-prey relationship between buzzards and red grouse. I have explored the diet, foraging patterns and responses to changing prey abundances of buzzards on a moorland site managed for red grouse in south-west Scotland. First, I investigated the biases associated with methods of assessing raptor diet. I demonstrated that methodological biases exist and that these can vary over time in relation to natural temporal variations in raptor diet composition. I then investigated functional and numerical responses of buzzards to annual changes in prey abundance. Following declines in vole abundance, buzzards selected a wider range of prey, but consumption of red grouse did not increase, and there was no evidence of a numerical response. Results suggested that buzzard predation of red grouse may be incidental in nature, whereby high vole abundances encouraged buzzards to hunt in red grouse habitats. Next, I explored buzzard foraging patterns in relation to prey and habitat. Buzzard foraging intensity varied in line with annual variations in vole abundance, and buzzards hunted in areas with more red grouse during the winter. Buzzards avoided heather dominated areas in years when vole abundance was low, but not when vole abundances were high. Results again suggested that incidental buzzard predation of red grouse could increase when vole abundances are high. However, I found no evidence that variations in buzzard foraging intensity influenced grouse mortality indices. iii I then described buzzard diet during the winter with the aid of remote tracking methods. Buzzard diet was primarily composed of small mammals, and red grouse were less likely to feature in the diet of buzzards roosting in grassy areas. Next, I produced estimates of the potential removal of grouse by buzzards using bioenergetics modelling. The results suggested that whilst the removal of grouse by an individual buzzard is likely to be small, the total number of grouse removed could be considerable if buzzard populations are high and predation of grouse is additive to other causes of mortality. Finally, key results are discussed and placed in a wider context of upland and gamebird management in Britain. Recommendations are made for future study to improve our understanding of these systems, and for testing possible mitigation and management techniques. This study could have wider implications for the management of economically important or threatened species, alongside recovering populations of protected raptors, and may provide a useful framework for studying similar systems elsewhere.