Factors affecting nest predation in the Great Crested Grebe: field observations, experiments and their statistical analysis

Abstract

Factors affecting predation by crows on nests of Great Crested Grebes (Podiceps cristatus) were studied during 1983-85 in Finland. The material consists of observations of grebes' nests and controlled experiments with artificial nests. During the study period, 39% of all grebes' nests were robbed at least once. The nesting success of natural nests did not depend statistically significantly on nest density. For the artificial nests used in the experiments, the occurrence of grebes' nests in the neighbourhood decreased the survival probability . The number of other artificial nests in the vicinity did not alone explain the observed degree of predation, butthe robbery of a nearby artificial nest had a strong effect, decreasing the survival probability . The most successful nests, natural or artificial, were those situated in very dense vegetation L90 reeds/m2 or in the vicinity of a gull's nest. The proportion of experimental nests destroyed by crows decreased when the egg was covered. By contrast, brown colouring of the eggs did not have any effect on predation . Logistic regression models for survival probabilities are given. The fact that in the experimental study the spatial distribution of nests is taken into account in the statistical analysis constitutes a methodological advance.