The effect of ice conditions in northern Hudson Bay on breeding by Thick-billed Murres (Uria lomvia)

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Abstract
We compared annual indices of breeding by Thick-billed Murres, Uria lomvia, at a colony in northern Hudson Bay with annual variation in the extent of sea-ice cover during May and June, the period when birds arrive and initiate breeding. Greater ice cover was associated with delayed breeding and lower attendance rates at the colony during the early incubation period (late June). We detected no effect on egg size, reproductive success, or adult body mass. The extent of ice cover in May was positively correlated with the proportion of Arctic cod fed to nestlings, and in June with the proportion of capelin in the rest of the diet. There was a positive correlation between the proportion of capelin in the chick diet and chick body mass at 14 days of age. We attribute the delay in laying during years of heavy ice cover to the reduced area available for foraging close to the colony. Prolonged persistence of ice, though delaying the onset of laying, may benefit the birds later in the season, as they forage preferentially in the vicinity of ice. We suggest that the strong relationships we observed between the extent of ice cover and aspects of the breeding biology of Thick-billed Murres may apply to other Arctic-nesting seabirds and may reflect conditions affecting the entire marine food web.