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 1km
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 slimmer, 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.