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UBC Theses and Dissertations

Factors affecting reproduction in great blue herons (Ardea herodias) Simpson, Keith


Reproductive success and size of 15 heronries were monitored from 1977 to 1980 in south coastal British Columbia. My main objectives were to inventory existing colonies, assess changes in colony status from historical information, and document factors important to reproduction. I collected data on banded herons at one colony to describe the movement of herons between and within heronries, and to identify the characteristics of individuals that related to reproductive success. Many heronries formerly identified were no longer present while others were at new locations or much larger in size. Heronry movements followed destruction of the nest trees or reproductive losses in several cases. Relocation normally occurred in the first or second year following heavy losses of young or adults. Disturbances by people sometimes forced herons to leave their nests and increased losses of eggs and young to predators. Severe predation continued after human disturbance had stopped. The number of young raised per successful nest was not a useful measure of reproductive success, since it varied little among colonies. The percentage of nests that succeeded, or numbers of young raised per breeding pair, provided better measures of reproductive success. Marked herons at one colony were not attached to specific nests or mates, and many adults probably switched colonies each year. Unsuccessful pairs did not renest in the same colony during the same breeding season. Although herons in central nests were more successful than those near the edge of the colony, central nests were not occupied by birds which were dominant on feeding areas. Herons are probably attracted to colonies to find new mates each year, and to reduce the vulnerability of their young to predators. Although 78 percent of the herons in one colony fed in the nearest feeding areas, many chose to travel further to feed. These distant feeders suffered higher nest losses to predators, probably because they left their nests unattended more often than other locally feeding birds. Some evidence suggested that males travelled further than females, and were less attentive at the nest. Males may play a dominant role in initiating colony relocations. The lack of attachment of herons to nest sites or mates helps to explain the changes in size and frequent movements of heronries in coastal B.C.

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