- Trinity University
Summary
In species in which both sexes have similar ornamentation, the ornaments often function as sexual or social signals in both sexes. However, males and females may use ornaments in different signalling contexts. We previously demonstrated that carotenoid-based bill colour of female American goldfinches, Spinus tristis, functions as a signal of status during intrasexual, but not intersexual, competition. Here we test whether male bill colour functions as a competitive status signal during both intra- and intersexual contests. We tested whether focal males and females avoided feeding adjacent to taxidermic male models as a function of the models' experimentally altered bill colour. We additionally tested whether male bill colour functions as a mate choice signal by presenting females with a choice of two live males with experimentally altered bill colour. In the status signal experiment, neither focal males nor females avoided male models with more colourful bills, as was predicted by the status-signalling hypothesis. These results indicate that male bill coloration does not function as a signal of competitive status and that the signal function of male bill colour does not parallel that of female bill colour. In our mate choice experiment, females showed no preference for male bill colour, suggesting that male bill colour may have some yet untested signalling function or that male bill colour may no longer be under selection. Our findings suggest that selection can lead to different signalling strategies in males and females, even in species that express mutual ornamentation.
Methodology
American goldfinches are socially monogamous with biparental care. Individuals spend the nonbreeding months in mixed-sex social flocks (McGraw & Middleton, 2009) and frequently engage in brief competitive interactions while foraging communally in both the nonbreeding and breeding seasons (T. G. Murphy, personal observation). Aviary-based experiments have demonstrated that competition for access to food and other agonistic interactions occur both within and between sexes (Coutlee, 1967, Popp, 1987a). Although both males and females defend nest sites during the nesting season (Coutlee, 1967, Middleton, 1979, Stokes, 1950), neither sex defends all-purpose territories, and instead individuals of both sexes forage communally in temporally and spatially ephemeral food patches. As a consequence, they regularly compete with both familiar and unfamiliar individuals for food. Females appear to be more aggressive than males during the nesting period (Coutlee, 1967).
Approximately 2–3 months prior to nesting, bill colour changes from drab brown to rich orange in both sexes. During the breeding season, male and female bill colour is similar in orange coloration, with only moderate male-biased sexual dichromatism (mean ± SE: bill brightness: males: 0.266 ± 0.007; females: 0.222 ± 0.008; bill saturation: males: 0.248 ± 0.001; females: 0.246 ± 0.002; bill hue: males: 550.1 ± 0.957 nm; females: 546.2 ± 1.085 nm; Kelly, Murphy, Tarvin, & Burness, 2012). Orange bill coloration is in part carotenoid-based (Hill, Hood, & Huggins, 2009) and has been shown to reflect stress and to respond to a short-term immune challenge in both sexes (Kelly et al., 2012, Rosenthal et al., 2012) and to coccidiosis in males (McGraw & Hill, 2000; as yet untested in females). Bill colour is correlated with immunoglobulin and natural antibody levels in females, but not in males (Kelly et al., 2012).
We captured birds at traps baited with niger seed. Sex and age class were determined based on plumage (Pyle, 1997). Upon capture, we measured basic morphometrics, colour of the upper mandible and throat plumage. All measures were taken by T.G.M. Colour measures were taken with an Ocean Optics USB2000+ spectrometer and PX-2 pulsed xenon lamp (Ocean Optics Inc, Dunedin, FL, U.S.A.) with the probe held 90° to the colour patch. The probe was mounted in a holder that minimized ambient light and positioned the tip of the probe approximately 7 mm from the substrate. We quantified reflectance (R) as the percentage of light reflected off the bill compared with a Spectralon white standard (Labsphere, Inc., North Sutton, NH, U.S.A.), at 1 nm intervals across the avian visual range (320–700 nm). The white standard was kept in a housing that ensured that the probe tip did not touch the surface of the standard, thus preventing the transfer of oil and dirt from the substrate to the standard. The spectrometer was calibrated to the standard prior to measuring each patch. We calculated the mean reflectance of five measures, which were taken at haphazardly chosen locations on the colour patch. All measures of bill colour were taken within 1 h of capture because bill colour can change rapidly (Rosen and Tarvin, 2006, Rosenthal et al., 2012). Using mean reflectance curves, we calculated mean brightness (‘luminance’; mean R from 320 to 700 nm), hue (wavelength where R = (Rmax + Rmin)/2) and yellow saturation ((sum of R from 550 to 625 nm)/total R from 320 to 700) using the program RCLR v0.9.33 (Montgomerie, 2010); see Table 3.2 in Montgomerie (2006) for further details.