Red Grouse Populations on Experimentally Treated and Untreated Irish Bog

Abstract

SUMMARY (1) On blanket bog in County Mayo, densities of red grouse (Lagopus lagopus scoticus (Lath.)) averaged only 5 km-2 in spring, ranging from 0-12 km-2. Timing of breeding, clutch size and hatchability resembled those in denser populations in east Scotland, but egg predation and chick losses were heavier. The size of full grown broods was inversely related to adult density in the preceding spring, adult losses in summer were positively related to adult density in the preceding spring, and winter losses positively to density in the preceding August. Hence this Irish population appeared to be non-cyclic during the study, showing major differences from the delayed density-dependence or time-lag effects characteristic of the cyclic populations studied in east Scotland. (2) On flat bog with deep peat where the birds' main food, heather (Calluna vulgaris (L.) Hull) was scarce, fertilizing (with draining and fencing) increased the heather's production, nutritive value and ground coverage. Grouse density in spring increased five-fold. Grouse reared larger broods and concentrated on the treated plots. On a hill where heather occurred more abundantly but was heavily grazed, we fenced out sheep and cattle. Within a year, heather grew better and grouse colonized the hill.