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Harsh weather in spring presents energetic challenges to birds during migration and upon reaching the breeding grounds, and yet, birds often arrive well before breeding begins. We studied a population of Eastern Kingbirds in eastern Oregon from 2004 through 2007. Early arriving kingbirds faced the poorest weather conditions, and therefore we predicted that benefits of early arrival must exist to balance the presumed costs. Earlyarriving kingbirds were more likely to both acquire a high-quality territory and to replace nests after failure. Early-arriving birds also bred early, and early breeding led to larger clutches and greater production of young. Earlyarriving males also sired more extra-pair young than later arrivers. Our data suggest that arrival date is in part influenced by individual quality, and that arrival date has reproductive consequences, with the primary benefits of early arrival being the acquisition of a high-quality territory, early breeding, and increased probability of replacing failed initial nests.

Theory predicts that maximal fitness is obtained by individuals who begin to breed immediately upon reaching sexual maturity. However, delayed breeding occurs regularly in some taxa, and in birds and mammals is most often associated with long lifespan and/or limited access to suitable habitats. Delayed breeding is not expected among relatively short-lived species such as migratory passerine birds, but this assumption remains untested in many species. Here we quantify age at first reproduction in an eastern kingbird Tyrannus tyrannus population breeding in an ecological island, and through both observational and experimental approaches, investigate the potential causes for the high frequency of delayed breeding that occurs in this population. Nearly half of the fledged nestlings that returned to the breeding grounds did not breed in their first potential breeding season. Some non-breeders occupied territories, for at least some period, but most remained as non-territorial 'floaters'. Parentage analysis failed to show any reproductive success for female floaters, and only limited success for male floaters, indicating that floating was not a successful reproductive tactic. On the other hand, a strong negative relationship existed between population size and the proportion of young birds that bred in their first year, and non-territorial birds of both sexes quickly filled territory vacancies created by experimental adult removals. Limited breeding habitat and territorial behavior of older birds thus appear to be the main causes of delayed breeding in kingbirds. The frequency of delayed breeding in most species is unknown but of potential significance because failure to incorporate accurate estimates of age at first reproduction in population models may lead to flawed population projections.