Ardea
Official journal of the Netherlands Ornithologists' Union

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Watson A. & Moss R. (1980) Advances in our understanding of the population dynamics of Red Grouse from a recent fluctuation in numbers. ARDEA 68 (1-4): 103-111
Previous research on Red Grouse populations in eastern Scotland had shown that the density of the spring breeding stock was limited mainly by the size of territories taken by cocks in the previous autumn. After springs when food was good, chicks survived well and young cocks in the following autumn took smaller territories than old birds, and vice versa after poor spring food. However, recent work on a population fluctuation in the 1970s showed no deterioration in spring food' supplies, and yet a major population decline occurred. During the successive years when densities increased in the early part of the cycle, the young cocks became increasingly unsuccessful in terms of the proportion that got territories, the size of the territory that they took, and the proportion that had a hen paired with them. At high density, the sex ratio in the territorial population showed a big excess of cocks, and a high proportion of adult grouse emigrated in spring and summer and returned in autumn without their young; these effects increased during the course of the decline. In addition, during the decline a large proportion of birds which had taken territories in autumn gave up their territories in late winter, and then died, whereas previously we had found that nearly all territorial birds survived till the breeding-season. An attempt to halt the decline by fertilizing an area of heather failed. Thus, although changes in food supplies had been sufficient to cause previous observed changes in the numbers of Red Grouse, the evidence suggests that they were not necessary for the recent decline. Changes in spacing behaviour at different densities and in different phases of the population cycle offer the best general explanation of Red Grouse cycles.


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