Demographic Causes and Predictive Models of Population Fluctuations in Red Grouse

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Abstract
This paper documents demographic causes of population change in red grouse [Lagopus lagopus scoticus] at a moor in northeast Scotland [UK] in 1961-1978. A fluctuation of .apprx. 2-fold amplitude in breeding densities occurred in 1963-1969, and a 5-fold one in 1969-1977. Grouse tended to breed well in years of increase and more poorly in years of peak and decline. Numbers did increase after 1 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. 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 yr of low numbers and net winter immigration after 2 summers of poor breeding. Intrinsic processes within the population caused losses more by emigration and extrinsic processes imposed on the population more by death. The ratio of hens to cocks in spring, the extent of summer emigration, the number of chicks per hen in Aug. 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. Winter loss could not be used to predict spring numbers because it could be calculated only after the spring counts. Variation in spring numbers 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 Aug. Three measures in a chick production ratio (the ratio of 1/2 the young birds present in Aug. to the number of adult cocks in spring, and similarly for hens) were combined. Changes in spring numbers could be predicted from the observed chick production ratio each year. Since the ratio was related the spring numbers 2 yr previously, predictions could be made and models built predicting grouse numbers several years ahead. 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. During the decline phase of the 2nd population fluctuation, brood size was lower than in the increase, although not so poor as in the decline during the 1st fluctuation. Modeling showed that without emigration in summer the reductions in brood size would not have been sufficient to cause the 2nd decline. The models were tested by retrodicting changes in numbers for years at the beginning and end of the study (the data for which had been deliberately excluded from the models) and by predicting numbers successfully on a different study area.