Starting over: experimental effects of breeding delay on reproductive success in early-arriving male American redstarts


A. E. McKellar, Dept of Biology, Queen's Univ., Kingston, ON K7L 3N6, Canada. Present address for AEM: Dept of Fisheries and Wildlife Sciences, Univ. of Missouri, Columbia, MO 65211, USA. E-mail:


Birds that arrive and breed early often have higher reproductive success than late individuals, either as a consequence of timing-specific advantages (the timing hypothesis) or because these individuals and/or their resources are of higher quality (the quality hypothesis). In this study, we examined the potential influence of several factors affecting reproductive success by experimentally delaying breeding of early-arriving male American redstarts Setophaga ruticilla, a species for which early male arrival is strongly related to increased reproductive success. Our manipulation involved the capture, holding, and release of males following pairing and territory establishment, resulting in the majority of subjects (67%) losing their initial mate (47%) or mate and territory (20%) and forcing them to start over approximately 12 d after their initial arrival. Males forced to start over (i.e. those losing their first territory and/or mate) did not experience any decrease in body condition, nor did their reproductive behaviour differ from that of early-arriving control males. We found that naturally early-arriving but experimentally manipulated males suffered reduced fledging success in comparison to early-arriving males that bred early or late, but equivalent success in comparison to males that arrived and bred late. Based on our results, we propose that the relationship between early arrival and higher reproductive success in this species is mediated not simply by individual male quality or absolute arrival timing alone, but rather some other aspect of resource quality is likely important. We discuss and present evidence for two alternative explanations under the quality hypothesis: female quality and territory quality. To our knowledge, our study is the first to investigate the effects of experimentally delaying male breeding time, strengthening previous correlational evidence for resource quality as a potentially important selective agent driving early arrival in migratory birds.