Surveys were conducted at 35 Great Blue Heron (Ardea herodias) colonies in urban and rural landscapes of south-coastal British Columbia, Canada, to investigate low nesting productivity during 1998 and 1999 compared to a decade ago. Nesting failure was common (59% of 1,247 nests) and widespread (in 90% of 31 colonies) and accounted for 96% of the variation in nesting productivity among colonies in 1999. Nesting failure was more frequent in small colonies (<50 nests) than in large colonies (≥50 nests). Human disturbance has probably increased due to growth in the human population in the region, and direct disturbance from the Bald Eagle (Haliaeetus leucocephalus) was more frequent in 1999 than a decade earlier. Eagles predated eggs, nestlings, and fledglings and were probably responsible for most of the observed nesting failure. Direct disturbance by humans was infrequent, but heron nesting productivity was negatively correlated with the frequency of human pedestrians near colonies. We suggest that Great Blue Heron breeding failure was more frequent in 1998 and 1999 compared to a decade ago because of the combined effects of human disturbance from land development and an increased frequency of eagle predation.
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Vol. 27 • No. 3