A population of 200 Common Loons Gavia immer nesting on a little-disturbed oligotrophic lake in northeastern Saskatchewan was studied from 1973 to 1975. The reproductive success, causes of nesting failure, pollutant content and shell thickness of eggs were reported and compared. Forty-six percent of pairs attempting to breed successfully raised young in any one year; a mean of 0.73 young were fledged per nesting pair. Thirty-eight percent of all eggs hatched. Predators, unexplained disappearance and inundation were the major causes of egg loss. Eggs contained significant levels of DDE, Dieldrin, PCBs and mercury; however 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.
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