Variable reproductive success of bald eagles on the British Columbia coast

To determine factors affecting productivity of bald eagles (Haliacctus leucocephalus) nesting near industrial sites in the Strait of Georgia, British Columbia, we compared their reproductive success to sites on the outer Pacific Coast. Mean production (fledged young per occupied territory) over the period 1992-95 was higher at sites in the Strait of Georgia, including the lower Fraser River Valley (1.2) and southeast Vancouver er Island (0.95), compared to subpopulations at putative reference areas at Clayoquot Sound (0.27) on the west coast of Vancouver Island and in northern Johnstone Strait (0.30: P < 0.001). Low breeding success was mainly the result of nest failure when no eggs were laid, or eggs were abandoned during incubation. Food supply apparently was the key factor limiting breeding success. Total lipids measured in nestling plasma samples varied among sites (P < 0.010), and there was a relation between mean plasma lipids and mean annual productivity (r(2) = 0.52, P = 0.043), suggesting that food was Limiting in areas of low productivity: even at successful nests. There were no differences in prey delivery rates to nests in the less productive Barkley Sound (0.56) compared to the more productive Fraser River Delta (1.1). There was, however, a difference in prey composition between the 2 sites (P < 0.001), with more small fish and few er birds and mammals brought to Barkley Sound nests. Mean hatching date was later at nests outside the Strait of Georgia but was not a determinant of productivity because we recorded the latest mean hatching date at Langara Island, a site of seasonal food abundance and fair productivity (0.61).