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1. We compared the reproduction of a marine diving bird, Brunnich's guillemot (Uria lomvia), breeding at two Arctic colonies close to the northern and southern limits of the species' range in the Canadian Arctic. 2. At both colonies, timing of breeding for Brunnich's guillemots was positively correlated with summer ice cover, which was determined by winter and spring temperatures. Spring temperatures also modified the effects of ice conditions on timing of breeding. 3. At Coats Island, northern Hudson Bay, in low Arctic waters, the date of egg-laying has advanced since 1981, simultaneous with a decrease in summer ice cover in surrounding waters. Lower ice cover in this region is correlated with lower chick growth rates and lower adult body mass, suggesting that reduction in summer ice extent is having a negative effect on reproduction. 4. Conversely, at Prince Leopold Island, in the High Arctic, there has been no …