Behaviours influencing the distribution of great blue herons (ardea herodias fannini) in the strait of Georgia, British Columbia

The Great Blue Heron (Ardea herodias fannini), a nonmigratory subspecies endemic to the Pacific Northwest of North America, is COSEWIC-listed in Canada as a species of 'special concern'. The Breeding Bird Survey (BBS), conducted since 1969, indicate an ongoing decline in numbers. Recent findings show that nest and colony abandonments are common, and are strongly associated with disturbance by bald eagles (Haliaeetus leucocephalus). Vanishing bearings of herons departing foraging sites were strongly associated with colony locations, and led to the discovery of small, previously unknown colonies. A probabilistic model predicted that dispersed nesting offers increased nestling safety under high eagle depredation. Ideal free distributions accounting for colony locations as well as foraging-site size and quality best matched the observed distribution of foraging herons. These findings support the hypothesis that great blue herons have redistributed into smaller, more widely-scattered colonies as eagle numbers have recovered over recent decades.