The Effect of Hen Harriers and Other Predators on Red Grouse Populations in Scotland.
Red grouse populations were monitored for three years on study areas in Speyside and Perthshire (Scotland), where numbers have generally been declining since the mid 1970's. These low density populations are thought to be especially vulnerable to extrinsic factors, such as predation. The aim of this research was to discover what effect predators were having on grouse populations throughout the year.
Adult grouse mortality, examined by searching for corpses on six 1km2 study sites, was concentrated in the winter months. Birds killed were in good condition and a greater proportion were found dead than in any other similar study. Peregrine falcons and red foxes were the main predators involved. Initial evidence suggested that the grouse killed were, at least partly, territorial birds and not surplus individuals. Both the number of raptors and the percentage of grouse killed was inversely related to density. Low density populations were maintained by net immigration.
In the summer, the behaviour of the hen harrier, both at the nest, and whilst hunting was examined. Harriers exhibited a type II functional response to grouse chicks, which appeared to be their preferred prey. Moors with breeding harriers produced fewer young grouse, and on those moors where harrier predation was examined, it could account for the bulk of the losses. On the study areas, it was estimated that harriers took up to 32% of the grouse chicks in 8 weeks.
A simulation model was produced which suggested that harriers can reduce the numbers of grouse shot, although there was a high degree of variation in shooting levels. The impact of harriers on subsequent grouse spring numbers was dependent on the level of overwinter losses, variation in harriers and possibly alternative prey densities.
Further research is necessary to clarify some issues, but it was apparent that the effects of predators would be greatest on low density grouse populations. At these low densities, their impact will depend upon grouse breeding success and the degree of overwinter mortality relative to movement between estates. The importance of conserving heather moorland, to protect both raptors and grouse, was stressed.