The Effects of Illegal Killing and Destruction of Nests by Humans on the Population Dynamics of the Hen Harrier Circus cyaneus in Scotland

Abstract

Breeding productivity, natal dispersal and survival of hen harriers Circus cyaneus were studied between 1988 and 1995 on moorland managed for sport shooting of red grouse, other heather moorland and young conifer forests in the uplands of Scotland. Nest success was much lower on grouse moors than on other land management classes. Annual productivity was 0.8 fledglings breeding female -1 year -1 on grouse moors compared with 2.4 on other moorland and 1.4 in young conifer forests. Human interference was recorded on half of the grouse moor estates studied and accounted for at least 30% of breeding failures in this land management class. It was much less frequent in the other land management classes. Annual survival of female hen harriers which bred on grouse moors was about half that of females breeding on other moorland. On grouse moors, survival of females which bred unsuccessfully was much lower than that of females which reared at least one fledgling. Survival of breeding females on other moorland was high and unrelated to breeding success. The difference in survival of breeding females between grouse moors and other moors was attributed to killing by humans. On average, 55-74 females were killed each year, 11-15% of the total population of breeding females in Scotland, excluding Orkney. The population of breeding females on grouse moors was estimated to decline rapidly without immigration. Harriers breeding on the other habitats were producing a surplus of female recruits approximately sufficient to compensate for the losses on grouse moors. Most females started to breed at I year old and most males at 2 years old. The percentage of breeding males which were I year old was higher on grouse moors than on the other land management classes. The median natal dispersal distance of both sexes exceeded 10 km. Harriers fledged from one land management class were often found breeding in another. Natal dispersal resulted in net movements of 1-year-old females between land management classes which were sufficient to reduce the differences in population trend which would otherwise have occurred. Moorland managed for grouse shooting was a sink habitat which received two-thirds of its female recruits from other habitats. The difference in productivity and survival between grouse moors and other habitats was attributed to illegal human interference. It is speculated that, without persecution, the hen harrier population in Scotland would increase, initially by about 13% year -1 , until a new, but unknown, equilibrium level was reached.