Influence of habitat on breeding performance of Hen Harriers Circus cyaneus in Orkney

Declines of many bird populations have been linked to a reduction or degradation of their preferred habitats, leading to reductions in breeding productivity (Donazar et al.1993, Tella
et al. 1998, Robinson et al. 2001, Schmiegelow & Monkkonen 2002, Browne et al. 2004, Fernandez et al.2004, Julliard et al. 2004). Although the precise mechanism linking habitat with productivity is often not understood, habitat may limit productivity when certain features are needed for nesting (e.g. Potts 1986, Bradbury & Bradter 2004), or are linked to prey abundance (e.g. Potts 1986, Tella et al. 1998, Benton et al. 2002). Where such declines occur, conservation measures aim to improve these habitats for the target species. However, it is important, before any habitat modification is to be implemented, to be able to predict the outcome of increasing preferred habitats on the breeding performance of the target species. When experimental manipulations of habitat are not feasible, one way to achieve this is to evaluate indirectly whether habitat influences breeding parameters, such as breeding density or performance.