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Recent switch by the Great Blue Heron Ardea herodias fannini in the Pacific northwest to associative nesting with Bald Eagles (Haliaeetus leucocephalus) to gain predator protection

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The Great Blue Heron Ardea herodias fannini Chapman, 1901 in the Pacific northwest appears to have modified nesting behaviour in response to the strong recent recovery of the Bald Eagle (Haliaeetus leucocephalus (L., 1766)) 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. We 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 250 m 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 m of eagle nests with high reproductive success. These herons had greater reproductive success than those nesting far from eagle nests.
Comparisons of nesting success (number of fledglings per active nest of the Great Blue Heron Ardea herodias fannini ) close to and far from active Bald Eagle ( Haliaeetus leucocephalus ) nests. Error bars are 95% confidence intervals and the broken horizontal lines are the overall mean values. The comparison indicates that predator protection is worth 0.5–0.7 fledglings per active nest. Abbreviations along the x axis are as follows: Other EA 05 and Other EA 06, other herons located within 200 m of an active eagle within 2005 or 2006, respectively; CFB 05 and CFB 06, the Chilliwack Canadian Forces Base heron colony in 2005 or 2006, respectively; Other Non EA 05 and Other EA 06, other nesting herons without an eagle nests within 200 m of the colony in 2005 or 2006, respectively. Points grouped and labelled as “Eagles 05/06” are based on nests in colonies with eagles. Points grouped and labelled as “No eagles 05/06” indicate nests in colonies without eagles. The points labelled “Eagle lost 06” is the colony at CFB Chilliwack, in which the eagle nest occupied in 2005 blew down at the start of the 2006 heron breeding season (see text). In pairwise comparisons, nesting success is significantly greater in nests close to than far from eagles in both 2005 (Student’s t test, t = 3.34, P < 0.001) and 2006 ( t = 8.44, P < 0.001). Heron nesting success was higher in the CFB Chilliwack colony than in other colonies far from eagle nests ( t = 2.10, P < 0.05) in 2005, but lower than in other colonies close to eagle nests in 2006 ( t = 4.03, P < 0.001). Analysis based on nesting success measures in 324 (2005) and 471 (2006) heron nests measured within the same 15 colonies in each year.
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