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' growth was reduced in years when the counts of attending adults peaked early in the season and early relative to the date of hatching. These observations suggest that the timing of breeding is not advancing to keep pace with changes in the timing of events in the arctic marine environment, leading to greater difficulty in provisioning nestlings. We also demonstrate a relationship between the state of the North Atlantic Oscillation and both the date of peak colony attendance and the growth of nestlings. This relationship suggests that large-scale ocean-atmosphere interactions influence the availability of prey for murres, although the mechanism by which this occurs is not yet understood. Our results support the idea that mismatching of avian breeding cycles with peaks in food abundance is an important consequence of global climate change.