We report on variation in rates of egg loss among Brünnich’s Guillemot Uria lomvia breeding at Coats Island, northern Hudson Bay, in 1997–1999, and on several cases of adult mortality during incubation in the same years. Common factors in the dates of peak egg loss and adult mortality were high maximum daily temperatures and the presence of high numbers of mosquitoes in the area. Mortality was confined to breeding sites close to the edge of the colony, where mosquito parasitism was highest, and to those sites exposed to afternoon sunshine. High temperatures that occurred on days without mosquitoes were not associated with high egg losses or with adult mortality. Hence, it appears that a combination of heat and mosquitoes was necessary to bring about observed mortality and egg losses. The dates of first appearance and peak abundance of mosquitoes at Coats Island have advanced since the mid-1980s, perhaps in response to ongoing climate change. The effects on breeding Brünnich’s Guillemots suggest that the birds have not had time to adjust their behaviour to the resulting changes in the timing of peak mosquito parasitism.