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The breeding schedules of birds may not change at a rate sufficient to keep up with the current pace of climate change, causing reduced reproductive success. This disruption of synchrony is called the “mismatch hypothesis.” We analyzed data on the breeding of Thick-billed Murres (Uria lomvia) at a colony in northern Hudson Bay, Canada, to examine the relative importance of matched and mismatched timing in determining the growth rates of nestlings. From 1988 to 2007 the date of break-up and 50% clearance of sea ice in surrounding waters advanced by 17 days, and the date on which the count of murres at the colony peaked, an index of food availability, advanced by the same amount. However, the median date of egg-laying advanced by only 5 days so that the number of days between the date of hatching and the date of peak attendance and 50% ice cover increased over the study period. Nestlings' …