Ardea
Official journal of the Netherlands Ornithologists' Union

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Cavé A.J.A. (1985) Winter severity and breeding bird numbers in a Coot population. ARDEA 73 (2): 129-138
This paper deals with the local survival rates of breeding Coots and their chicks in a study area near Aalsmeer, The Netherlands. The terms local survival and local mortality are used, since we are unable to distinguish between mortality and permanent emigration. Only a proportion of the first-year birds, depending on density of breeding birds, are able to establish a territory. Once established, the breeding birds have a territory each year. The local survival rates of the three groups of birds distinguished (male and female birds with breeding experience, and first-year birds) depend on the severity of the winter as measured by the mean number of ice days at the five main weather stations in The Netherlands. The susceptibility to winter cold is not significantly different for the three groups of birds. The local survival rates of the first-year birds are distinctly lower than those of the breeding birds. To study the effects on the pattern of breeding bird numbers by mortality related to winter severity, population models were developed to simulate these numbers. These models show that winter severity alone is not responsible for the observed pattern (Fig. 2A), nor is (delayed) density dependence (Fig. 2B). It was concluded that the observed pattern is largely the result of the interacting effects of varying winter mortality and a delayed density-dependent factor (Fig. 2C). It was assumed that density dependence results from territorial behaviour, while the time lag was thought to be brought about by differences in competitive power related to breeding experience.


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