Associative nesting behaviour between Pacific great blue herons and bald eagles in the Pacific Northwest: testing the predator protection hypothesis

Pacific Northwest great blue herons (Ardea herodias fannini) appear to have modified nesting behaviour in response to the strong recent recovery of the bald eagle (Haliaeetus leucocephalus) population. Previously undescribed, herons now often nest in close association with some breeding eagles, even though eagles depredate heron nestlings, are implicated in the recent reproductive decline of herons, and may induce abandonment of heron breeding colonies. I tested the hypothesis that breeding herons gain protection from the territorial behaviour of eagles. Natural observations and simulated incursions showed that nesting eagles actively repel other eagles within at least 250m around the nest site, thereby establishing a relatively safe place for herons to nest. Surveys showed that 70% of heron nests and 19% of heron colonies were located within 200 metres of eagle nests with high reproductive success. These herons had greater reproductive success than those nesting far from eagle nests.