Correlates and consequences of coloniality in great blue herons

Abstract

Sixty percent of the breeding great blue herons (Ardea herodias) in a colony at Pender Harbour, British Columbia, were individually marked to test three hypotheses concerning the benefits of colonial breeding. (i) Most herons fed near the site of the colony, but neighbours or members of a pair did not tend to choose adjacent feeding sites, as predicted by the information center hypothesis. Food supply was predictably distributed in time and space, and birds that did most of their feeding outside Pender Harbour bred less successfully than locally feeding birds. (ii) Nests that failed because of predation were nearer to the edge of the colony in 1978 but not in 1979. The colony was abandoned in 1980 after 2 years of high predation. Predation may, therefore, select for colonial breeding in herons, but heavy predation may force colony abandonment. (iii) Most herons chose new nest sites and new mates each year. The colony site may therefore serve as an assembly site for mate finding. Mate finding and avoidance of predation are therefore more likely to explain coloniality in great blue herons than are advantages associated with finding food.