THE IMPACT OF HEN HARRIERS ON RED GROUSE BREEDING SUCCESS

(1) The effect of predation by hen harrier (Circus cyaneus L.) on the breeding success of red grouse (Lagopus lagopus scoticus Lath.) was examined on managed moorland in Scotland. (2) Extensive studies of breeding success on pairs of moors indicated that moors with breeding harriers produced an average of 17% fewer young grouse. On one pair of moors losses varied positively with harrier density over 3 years. (3) On intensive study areas, grouse brood size was measured 2 weeks after hatching and 6 weeks later. Harriers were observed hunting during this period and a figure for expected chick losses obtained. Over the 3 years harrier predation accounted for 91% of observed losses in the 6 weeks. (4) Prey was examined at three harrier nests in 1988. Of 299 items identified, 32% were grouse chicks, although there were significant differences among nests. (5) Data from this and other studies of hen harrier diet were collated. The relationship between grouse density and the proportion of grouse in the harrier diet indicated that grouse populations at low density are most vulnerable to high chick losses.