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Résumé :
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1. This paper describes the results of a five-year population study of red grouse on selected study areas in north-east Scotland, chiefly on two study areas of 460 and 405 ha, but also on smaller areas elsewhere. 2. Counting the grouse, many of which were marked, at regular intervals showed a decline on the main study areas 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. 3. At all population densities, grouse numbers were apparently limited by the birds' behaviour, probably in relation to their food supply. Different densities were tolerated with different behaviour patterns, higher densities being permitted 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'. 4. 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. 5. 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. 6. 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 three good breeding years with four or more young per pair in August, and two poor years with two or fewer young per pair. There was a significant correlation between clutch size, proportion hatching and chick survival over the five years; and good breeding success was correlated with good adult survival, heavy adult weights in August, 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. 7. Annual changes in both spring numbers and breeding success were associated with changes in the main food in the previous winter.
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