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Keywords:

  • Bayesian model;
  • field voles;
  • meadow pipits;
  • population dynamics;
  • red grouse;
  • super-population model;
  • wildlife management

Summary

1. The impact of hen harriers Circus cyaneus on red grouse Lagopus lagopus scoticus populations has received much attention. However, little has been done to model the population dynamics of the hen harrier alone. Such a model is needed to help inform the differing aims of conserving harriers and managing grouse moors, which serves as a reflection of human–wildlife conflicts around the globe.

2. On Langholm estate in Scotland, intensive studies have resulted in harrier numbers being known without error. We fit a Bayesian population model to these data, using a super-population model to permit inference in the presence of demographic and environmental stochasticity and in the absence of observation error.

3. Hen harriers have a straightforward life history. After fledging, juveniles show little natal site fidelity, often dispersing long distances into breeding areas rich in their preferred prey, the field vole Microtus agrestis and meadow pipit Anthus pratensis. Therefore, any increase in a local population is largely because of recruitment into the area as opposed to fledging success. Once birds have settled in an area, harriers are generally site faithful, with year-to-year survival depending, in part, on the density of meadow pipits.

4. Our model suggests that temporal patterns in harrier numbers on managed grouse moors, in the absence of illegal persecution, are influenced by vole numbers, whereas meadow pipit density appears to have a limited effect.

5.Synthesis and applications. Our modelling approach is a useful way to infer population processes, and the effects of the environment on these processes, for populations censused without error. When used to predict future harrier numbers under alternate management scenarios, our model indicates that harrier numbers on Langholm estate, Scotland, could be reduced without any direct human intervention if the estate can be managed in a way that reduces vole populations. In contrast, there appears little to gain from managing meadow pipit densities. If these conclusions apply to other harrier populations, then management to reduce vole numbers, while maintaining grouse densities, may help alleviate the conflict between conservationists and managers of grouse moors.