Population Studies on Red Grouse, Lagopus lagopus scoticus (Lath.) in North-East Scotland

Abstract

Red grouse, many of them marked, were counted at regular intervals over 5 years. They showed a decline on the main study areas in northeast Scotland from a peak of about one breeding pair to 2 ha in 1957 and 1958 to about one pair to 6 ha in 1960, and an increase in 1961 to a peak similar to 1957. At all population densities, grouse numbers were apparently limited by the birds'' behavior, probably in relation to their food supply. Different densities were associated with different behavior patterns, higher densities occurring when the birds were in family parties or flocks than when they were territorial. The number of breeders depended on the size of the territories. When all the breeding habitat was occupied, surplus birds without territories dispersed to marginal habitats where they died, mostly heavily parasitized and emaciated. Such birds are sometimes referred to as suffering from "grouse disease". The grouse population on the breeding area had three social components mated and unmated territory owners on optimum habitat, surplus non-territorial residents on marginal interspaces between territories, and surplus birds transient between other marginal areas where long-term survival was impossible but where grouse lived temporarily. Casualties among breeders were often replaced by previously surplus non-breeders, and mortality did not regulate the numbers of territory owners. Shooting exploited part of the surplus, and natural mortality mostly eliminated other birds that could not obtain or hold territories. Recruitment was adequate every year to stock the breeding areas to the levels permitted by the territory size next season. On the principal study areas there were 3 good breeding years with 4 or more young per pair in Aug., and 2 poor years with 2 or fewer young per pair. There was a significant correlation between clutch size, proportion hatching and chick survival over the 5 years; and good breeding success was correlated with good adult survival, heavy adult weights in Aug., and with 1:1 sex ratios in spring. Breeding success seems therefore to have been predetermined, and apparently it was not greatly influenced by the summer weather. There was no correlation between breeding success and spring numbers or the amount of previous dispersal. Annual changes in both spring numbers and breeding success were associated with changes in the main food in the previous winter.