In many avian species, reproductive success increases with parental age and experience, and declines seasonally. However, it is difficult to evaluate how parental attributes affect reproduction independently of date effects, because young, inexperienced pairs generally lay later than older, experienced pairs. We examined how parental experience and timing affected nestling growth in the Thick-billed Murre (Uria lomvia), an Arctic seabird that lays a single-egg clutch, by taking advantage of a natural contrast made possible by marked within-group synchrony. That is a common feature at murre colonies, and enabled us to monitor the breeding performance of experienced pairs over an extended period that overlapped with breeding by inexperienced pairs late in the season. Whereas growth of offspring raised by experienced parents was unaffected by their hatching date, offspring raised by inexperienced parents grew more slowly than those raised concurrently by experienced parents. Therefore, parental experience influenced nestling growth rates whereas timing did not, a result that accords with previous studies on this species. Absence of direct effects of timing of egg-laying on breeding success of Thick-billed Murres stands in sharp contrast to many other avian species, and seems surprising for an Arctic-nesting species often assumed to be strictly time-constrained in its breeding.
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