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Environmental contaminants, disturbance and breeding failure at a great blue heron colony on Vancouver Island Moul, Ian E.


Great Blue Herons (Ardea herodias) breeding near a pulp mill at Crofton B.C. failed to raise young in 1987 and 1988. Elevated levels of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-furans were detected in their eggs. The highest 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxic equivalent level observed was 496 ng/kg (wet weight) in 1987 (Elliott et al. 1989). I compared the Crofton colony with a less contaminated colony on Sidney Island in 1988 and 1989. I examined three possible explanations for nesting failure: (i) abnormal nesting behaviour by parents because of contamination, (ii) disturbance by human activities, and (iii) predation by Bald Eagles (Haliaeetus leucocephalus), Northwestern Crows (Corvus caurinus) and Common Ravens (Corvus corax). The Crofton colony was successful in 1989 and 1990. Within the TCDD toxic equivalent range of 34 - 257 ng/kg (1989), I observed no abnormal heron nesting behaviour or reduction in numbers of chicks fledged. In 1988 the herons at Crofton were disturbed repeatedly by human activities. Bald Eagles were observed daily at both Crofton and Sidney Island. The heron colony on Sidney Island failed in 1989 and 1990. The failures on Sidney Island were thought to involve a disturbance by eagles followed by rapid removal of eggs and young chicks at unattended nests by crows and ravens. It is likely that disturbance and predation played a part in the failure at Crofton in 1988, but there remains the possibility that environmental contaminants may have increased the sensitivity of herons to disturbance and predation.

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