The Adaptive Significance of Colonial Breeding in the Grey Heron Ardea cinerea: Inter- and Intra-Colony Variability in Breeding Success

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Abstract
We studied variations in reproductive success within and between Belgian heronries to evaluate the hypotheses that colonial breeding in Grey Herons is an anti-predator adaptation or an adaptation to facilitate food-finding. Breeding success of Grey Herons did not increase with number of breeding pairs present, nor with increasing synchronization of breeding, and pairs nesting in the centre of the heronry did not fledge more chicks than pairs nesting at the edge. We found no consistent differences in the timing of onset of reproduction and in clutch size between and within heronries. There was a suggestion that first-year breeding birds tended to occupy nests at the edge of a heronry, but their proportion in the breeding population probably was too small to affect the results. We suggest that the predation pressure is too low to cause anti-predator responses, such as mobbing, to evolve among herons. There was no evidence for a higher chick survival as a consequence of increasing information transfer through increased number of breeding pairs. The data suggested that new heronries establish near suitable foraging sites, but that overall reproduction success decreases when more pairs settle.

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