Summary
The study of sexual selection has traditionally focused on events and behaviours immediately surrounding copulation. In this study, we examine whether carry-over effects from the non-breeding season can influence the process of sexual selection in a long-distance migratory bird, the American redstart (Setophaga ruticilla). Previous work on American redstarts demonstrated that overwintering in a high-quality habitat influences spring departure dates from the wintering grounds, advances arrival dates on the breeding grounds and increases apparent reproductive success. We show that the mixed-mating strategy of American redstarts compounds the benefits of overwintering in high-quality winter habitats. Males arriving to breed in Canada from high-quality winter habitats arrive earlier than males from poor-quality habitats, resulting in a lower probability of paternity loss, a higher probability of achieving polygyny and ultimately higher realized reproductive success. Such results suggest that the process of sexual selection may be influenced by events interacting throughout the annual cycle.
Methodology
Fieldwork was conducted in May–July 2004–2007 at Queen's University Biological Station, Chaffey's Lock, Ontario, Canada (44°34′ N, 76°19′ W). Our study area is composed of mixed-deciduous forest, dominated by sugar maple (Acer saccharum) and Eastern hop hornbeam (Ostrya virginiana). When males arrive on the breeding grounds, they immediately begin singing for territory advertisement and to attract females. Each year, from May 1 to 31, we surveyed our 60 ha study area daily from 06.00 to 12.00, detecting males by the presence of singing and subsequent visual identification. Arrival date was standardized as the number of days after the first male arrived (first-male arrival date=0). All adults were captured in mist-nets within 7 days of arrival by simulating territorial intrusions using song playbacks accompanied with a decoy. Once captured, American redstarts were individually marked with a single Canadian Wildlife Service aluminium band and two to three colour bands. We then extracted 50 μl of blood for paternity analysis by piercing the brachial vein and clipped 2–3 mm of the central claw for stable-isotope analysis (2006 and 2007 only).
Upon arrival, all males were observed and mapped for at least 20–30 min d−1 throughout the breeding season to determine territory boundaries and pairing date. Females typically begin nest building within a few days of pairing. Once nest building began, we monitored nest status every other day, noting the onset of egg laying, number of eggs laid, hatching and fledging success. Males were monitored daily to detect individuals that paired with secondary females (i.e. polygynous mating). At day 5 after hatching, we banded nestlings with a single aluminium band and collected 15–20 μl of blood for paternity analysis. Offspring from nests that were too high to access on day 5 were captured on the day of fledging.
American redstart males exhibit delayed plumage maturation, wherein males resemble females during their first breeding season and do not mature into the full adult breeding plumage until their second prebasic moult, which follows their first breeding season. Owing to the differences in plumage, and the fact that the first-year American redstarts have greatly reduced reproductive performance (Sherry & Holmes 1997; M. W. Reudink 2007 unpublished data), we limited our analyses to only adult (after second-year or ASY) males.