Breeding Performance, Pollutant Burden and Eggshell Thinning in Common Loons Gavia immer Nesting on a Boreal Forest Lake

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Abstract
A population of 200 common loons G. immer nesting on a little-disturbed oligotrophic lake in northeastern Saskatchewan [Canada] was studied from 1973-1975. The reproductive success, causes of nesting failure, pollutant content and shell thickness of eggs are reported and compared. Of pairs attempting to breed, 46% successfully raised young in any 1 yr; a mean of 0.73 young were fledged per nesting pair. Of all eggs 38% hatched. Predators, unexplained disappearance and inundation were the major causes of egg loss. Eggs contained significant levels of DDE, Dieldrin, PCB [polychlorinated biphenyls] and mercury; no statistical relationship to reproductive performance was found. Shell thickness was inversely correlated, shell density directly correlated with DDE content. The mean shell thickness was identical to the pre-1947 mean although the variance was greater and some shells were 23% thinner. Altered shell density was interpreted as evidence of structural abnormalities. About 47 ppm DDE are necessary to induce 20% shell thinning in this species, but its large pore area may make it more sensitive to pollutant-induced alterations.