- University of Guelph
- Queen‘s University
Summary
Understanding the causes of variation in feather colour in free-living migratory birds has been challenging owing to our inability to track individuals during the moulting period when colours are acquired. Using stable-hydrogen isotopes to estimate moulting locality, we show that the carotenoid-based yellow–orange colour of American redstart (Setophaga ruticilla) tail feathers sampled on the wintering grounds in Central America and the Caribbean is related to the location where feathers were grown the previous season across North America. Males that moulted at southerly latitudes were more likely to grow yellowish feathers compared with males that moulted more orange–red feathers further north. Independent samples obtained on both the breeding and the wintering grounds showed that red chroma—an index of carotenoid content—was not related to the mean daily feather growth rate, suggesting that condition during moult did not influence feather colour. Thus, our results support the hypothesis that feather colour is influenced by ecological conditions at the locations where the birds moulted. We suggest that these colour signals may be influenced by geographical variation in diet related to the availability of carotenoids.
Methodology
Feather sampling
On the breeding grounds (May–June 2001–2004), we sampled tail feathers from male redstarts captured at the Queen's University Biological Station (44°34′ N, 76°19′ W) in southeastern Ontario, Canada. Individuals marked in year x were recaptured in year x+1 and a single tail feather (third retrix) was removed (Norris et al. 2004), thus providing an estimate of feather growth rate during the annual moult in year x. We also sampled a third retrix from after-hatch-year (AHY; second wintering season or older) and hatch-year (HY; first wintering season) males (n=122) at 12 locations throughout the wintering range (9.3°–32.2° N and 60.6°–105.7° W; figure 1a). We collected feathers in January–March 2001–2004, except from the Dominican Republic (1997) and Belize (1999).
Analysis of moulting region
We used stable-hydrogen isotopes (δD) to estimate moult location. Stable-hydrogen isotope ratios (2H/1H=R) are expressed in δ notation (‰) where δ=[(Rsample/Rstandard)−1]×1000 and Rstandard is the hydrogen isotope ratio of Vienna Standard Mean Ocean Water. For a detailed description of sample preparation and analysis, see Norris et al. (2006). δD values in feathers sampled on the wintering grounds (January–March) indicate moulting latitude the previous July–September. Norris et al. (2006) used a likelihood assignment method to estimate the moulting regions (figure 1a) of individuals that were sampled on the wintering grounds, based on expected δD values in precipitation and relative breeding abundance. We found no significant variation in δD values among years within localities (Norris et al. 2006), hence we pooled samples across years for each locality.
Feather colour analysis
We measured reflectance across the bird-visible spectrum (320–700 nm) of five haphazardly chosen areas within each yellow–orange colour patch on each tail feather using an Ocean Optics USB2000 spectrometer connected to a PX-2 pulsed xenon light source (Norris et al. 2004). From each spectrum, we calculated red chroma as the proportion of total reflectance from the orange–red region of the spectrum (575–700 nm) and calculated the mean for each individual. Red chroma is a measure of spectral purity in that part of the spectrum (Montgomerie 2006) and is correlated with the amount of carotenoids deposited in the feather (Saks et al. 2003).
Feather growth rate analysis
We digitally photographed each feather under a dissecting microscope with the feather illuminated at an oblique angle by a fibre-optic light source to reveal the faint growth bars that result from each day's growth (Hill & Montgomerie 1994). Using ImageJ (v. 1.35a; available at http://rsb.info.nih.gov/ij/), we then measured the total distance (parallel to the rachis) encompassed by three adjacent growth bars located about one-third of the distance from the feather tip. We also measured as many adjacent growth bars as possible on each feather. Images were enlarged 5× and the same part of each feather was measured to control for any differential growth during moult. To reduce measurement error, growth bars were measured once in each direction and an average was calculated. We used means from the maximum number of growth bars measured, assuming this to be more accurate as it is based on a larger sample. Our conclusions are the same for both measures.