Many animals display multiple signals that can be used by conspecifics to gather information about the condition or quality of potential mates or competitors. Different signals can indicate different aspects of individual quality or function in spatially or temporally separated periods. However, for long-distance migratory birds, it is unclear if signals, such as plumage traits, function in different phases of the annual cycle. We investigated the potential role of carotenoid-based tail and flank plumage, and bib size, in relation to extrapair paternity and polygyny in the American redstart, Setophaga ruticilla. This work complements our previous research suggesting tail feather brightness acts as a status signal, mediating territory acquisition during the nonbreeding season in Jamaica. Here, we show that tail feather brightness also serves as an important signal during the breeding season. Specifically, our results indicate that polygyny, a behaviour highly dependent on obtaining and defending multiple territories, is significantly predicted by tail brightness. Interestingly, flank redness best predicted whether individuals secured paternity at their nest and the proportion of within-pair offspring sired. We suggest that by expanding the study of plumage function in long-distance migrants to events occurring throughout the annual cycle, we gain a critical perspective on the function and evolution of ornamental traits.
Field work was conducted May–July 2005–2007 at the Queen's University Biological Station, Chaffey's Lock, Ontario, Canada (44°34′N, 76°19′W). Our study area was composed of mixed-deciduous forest, primarily dominated by sugar maple (Acer sacchaurum) and eastern hophornbeam (Ostrya virginiana). When males arrive on the breeding grounds, they immediately begin singing for territory advertisement and to attract females. During 1–31 May, we surveyed our 60 ha study area from 0600 to 1200 hours, detecting males by the presence of singing and subsequent visual identification. ASY males arrive first on the breeding grounds (mean ± SE = May 4 ± 1.4 days), followed by females (May 9 ± 1.7 days), then SY males (May 13 ± 2.8 days). Most ASY males were captured in mist nets within 7 days of arrival by simulating territorial intrusions using song playbacks (recorded at Hubbard Brook, NH, U.S.A.) accompanied with either a stuffed ASY male decoy or a caged live male (SY or ASY). Male redstarts at our study site are extremely aggressive and relatively easy to catch. Because we monitored the entire study site on a daily basis, we were able to ensure that we captured all males holding territories within the study area. Females were captured either by simulating territorial intrusions or by using song playback of fledgling alarm calls accompanied by a female decoy. Redstarts were individually marked with a single Canadian Wildlife Service aluminium band and two to three colour leg bands. We then extracted 50 μl of blood for paternity analysis by piercing the brachial vein. Consistent with our previous work, we used wing length (mm) as our measure for relative body size (Reudink et al., in press). From each individual, we plucked a single tail feather (third rectrix, R3) and 12–15 feathers (Quesada & Senar 2006) from the center of the orange portion of the flanks. We did not pluck any primary feathers (which also have a carotenoid-based orange patch) to lessen the invasiveness of our study.
Upon arrival, all males were observed and mapped for at least 20–30 min/day 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, and hatching and fledging success. Males continued to be monitored daily to ensure that we detected polygynous matings. At day 5 after hatching, we banded nestlings with a single aluminium band and took 15–20 μl of blood for paternity analysis. When nests were inaccessible, the offspring were captured on the day of fledging.
Because redstarts show delayed plumage maturation and few SY males pair and fledge offspring (one of 22 males that stayed in our study area throughout the entire season fledged a single offspring), we restricted our analyses to ASY males.