The impact of hen harrier (Circus cyaneus) predation on red grouse (Lagopus lagopus scoticus) populations: Linking models with field data.

Abstract

1. In the absence of human persecution the densities of breeding hen harrier Circus cyaneus on heather-dominated moorland appear to be determined by the abundance of passerines and small mammals.
2. Small prey are important in the diet of harriers in the spring, whilst the chicks of red grouse Lagopus lagopus scoticus form an important component of the diet during the summer. This predation brings harriers into conflict with grouse managers, who seek to maximise the number of grouse available for shooting in the autumn.
3. We build on published data on the functional response of harriers to grouse chicks and use a deterministic model to understand how the numbers of harriers breeding on grouse moors influence the average density of autumn grouse populations.
4. The model suggests that driven shooting could coexist with breeding harriers over a range of grouse brood sizes and harrier densities. If the effect of supplementary feeding on grouse predation rates observed at Langholm hold more widely across grouse populations, then the model suggests that driven shooting could occur across an even broader range of harrier densities. The model now needs to be subject to sensitivity analysis and its assumptions examined.