Demographic Causes and Predictive Models of Population Fluctuations in Red Grouse

Abstract

SUMMARY (1) This paper documents demographic causes of population change in red grouse at a moor in north-east Scotland in 1961-78. A fluctuation of about two-fold amplitude in breeding densities occurred in 1963-69, and a five-fold one in 1969-77. (2) Grouse tended to breed well in years of increase and more poorly in years of peak and decline. However, numbers did increase after one summer with poor breeding, and decreased after several summers with good breeding, so good breeding was not necessary for increases, nor poor breeding for declines. There were fewer hens than cocks in spring, and many fewer in some years of decline. (3) Many birds emigrated in summer during decline years. Winter losses were only partly due to deaths in the area, and in most years involved much emigration. Net summer immigration occurred in 2 years of low numbers, and net winter immigration after two summers of poor breeding. Intrinsic processes within the population caused losses more by emigration, and extrinsic processes imposed on the population more by death. (4) The ratio of hens to cocks in spring, the extent of summer emigration, the number of chicks per hen in August, and winter loss were all correlated with one another. Winter loss was the loss best correlated with changes in spring numbers, and the largest single loss. The change in numbers from spring i to i + 1 was correlated with numbers in springs i - 1, i- 2 and i - 3, and most strongly at i- 2. (5) Winter loss could not be used to predict spring numbers because we could calculate it only after the spring counts. Variation in spring numbers, however, could be accounted for by a multiple regression on the previous year's spring sex ratio, the extent of summer emigration, and the number of chicks per hen present in August. (6) We combined these three measures in a chick production ratio (the ratio of half the young birds present in August to the number of adult cocks in spring, and similarly for hens). Changes in spring numbers could be predicted from the observed chick production ratio each year. As the ratio was related to spring numbers 2 years back, we could predict it and so build models predicting grouse numbers several years ahead. (7) Models based entirely on demographic data, with no explicit environmental effect on numbers, fitted observed numbers reasonably well. The only observed extrinsic environmental relationship with numbers was the proportion of nests robbed. Models that incorporated this fitted observed data better. (8) During the decline phase of the second population fluctuation, brood size was lower than in the increase, although not so poor as in the decline during the first fluctuation. Modelling showed that without emigration in summer the reductions in brood size would