At least once a year, birds face the energetically demanding task of molting all their flight and body feathers. As a result, most birds avoid an overlap between molt and other costly activities during the annual cycle (e.g., raising young, migrating). Most Nearctic–Neotropical migratory birds undergo an entire prebasic molt at the end of the breeding season, before fall migration, and some also go through a second, pre-alternate molt of body feathers on the wintering grounds to refurbish their breeding plumage before reproduction (Pyle 1997, Froehlich et al. 2005). Even so, some birds employ a strategy of molt-migration, delaying some or all of their prebasic molt until after fall migration begins (Stresemann and Stresemann 1966). This phenomenon is relatively common in shorebirds, and nearly half the Neotropical migrants molt at least some feathers south of the breeding grounds (Leu and Thompson 2002). However, molt-migration is not nearly as frequent in passerines (e.g., only 7 of 53 wood warblers are reported to complete at least some of their prebasic molt outside of the breeding grounds: Vermivora peregrina, V. celata, V. luciae, Dendroica petechia, D. pensylvanica, D. kirtlandii, and Protonotaria citrea; Pyle 1997), and it also appears to vary geographically, occurring more commonly in western North America (Rohwer et al. 2005).
Breeding population.—Work on the breeding grounds was conducted May–July, 2004–2007, at the Queen's University Biological Station, Chaffey's Lock, Ontario (44°34′ N, 76°19′ W), using the same study population of American Redstarts as (Norris et al. 2004b) and (Langin et al. 2007). All birds were captured and processed following (Norris et al. 2004b). One difference, however, is that none of our nests was experimentally depredated as was done with a subset of birds by Norris et al. (2004b; see Langin et al. 2006). We also collected the first primary (P1) from all individuals and a second rectrix from individuals with regrown tail feathers (i.e., feathers that appeared less orange than the rest of the feathers, were unworn and had narrow growth bars).
Overwintering population.—American Redstarts were also studied at a wintering site consisting of high-quality (dominated by Black Mangrove [Avicennia germinans]) and low-quality (second-growth scrub dominated by Logwood trees [Haematoxylon campechianum]) habitats from January to March, 2004 and 2005, at Font Hill Nature Preserve, Westmoreland Parish, Jamaica (18°02′ N, 77°57′ W; see Marra 2000). We studied birds overwintering in Jamaica because populations breeding in the northeastern United States and southern Ontario appear to overwinter in the Caribbean (Norris et al. 2006), which suggests that conditions in Jamaica should be similar to those experienced by our breeding population during the non-breeding season.
We tested the hypothesis that birds previously reported as molt-migrants may have lost and subsequently regrown tail feathers on the wintering grounds and that feather regrowth may be condition-dependent. To do this, we captured birds in Jamaica during January–February, weighed them to the nearest 0.1 g, and plucked a single tail feather (R3). Before spring departure, we recaptured the same individuals to determine overwinter change in body mass and whether they regrew the plucked feather. Feathers were defined as "regrown" if they extended >1 cm from the feather sheath. For a subset of birds (n = 10 ASY males), we then plucked the regrown feather for comparisons of δD and color between the original and regrown feathers.
Stable-hydrogen-isotope and color analyses.—Details of our stable-hydrogen-isotope analysis are reported in (Langin et al. 2007) and are the same as those employed by (Norris et al. 2004b). Reflectance spectra from tail feathers were obtained by measuring percent reflectance from 320–700 nm using an Ocean Optics USB2000 spectrometer attached to a PX-2 xenon pulsed light source. The sheathed probe was held at a 90 angle to the feather surface. All feathers were mounted on minimally reflective (<5% reflectance) black paper (Colorline no. 142 Ebony). To standardize our measurements, we took dark and white (spectralon) standard readings between each measurement and the next. Twenty-five measurements were taken haphazardly within the yellow–orange region of each tail feather, avoiding the rachis. We then averaged across the 25 reflectance spectra and quantified plumage coloration by calculating standard measures of brightness, hue, and chroma (Montgomerie 2006). Our measure of red chroma is the same as that reported by (Norris et al. 2004b): brightness = mean R320–700, hue = arctan([(R415–510 − R320–415)/R320–700]/[(R575–700 − R415–575)/R320–700]), UV chroma = R320–415/R320–700, and red chroma = R575–700/R320–700.