- Trinity University
Summary
Status signals are linked to fighting ability and enable competitors to gain access to resources without risking injury in aggressive combat. The relationship between testosterone (T), a hormone that mediates aggression, and signals of status is well studied in males, but little is known about the relationship between T and female signals of status. Female and male American goldfinches Spinus tristis express a dynamic carotenoid‐based orange bill color during the breeding season and previous work has demonstrated that females use orange bill color to communicate competitive ability during intrasexual competition. We test the hypothesis that female bill color reflects baseline T, which would allow receivers to directly assess a competitor's aggressive potential. We found a positive relationship between T and bill coloration in females, indicating that bill color has the ability to signal female competitive status. This finding is consistent with the hypothesis that female bill color is a reliable signal of fighting ability, and indicates that females, like males, may use coloration to signal their hormonally mediated aggressive potential.
Methodology
Species
The American goldfinch is a socially monogamous passerine with bi‐parental care, and typically produces a single brood per season (McGraw and Middleton 2009). In southern Ontario, where our study was conducted, pairs initiate their first clutches in early July. Both males and females are aggressive in this species: both sexes defend an area immediately surrounding the nest (Stokes 1950, Coutlee 1967) and compete for food in communal feeding areas (Popp 1987). Females appear to be more aggressive than males during the nesting period (Coutlee 1967), although the ecological contexts of this female‐biased dominance are unclear.
General
This study was carried out in southern Ontario, Canada, near the Queens Univ. Biological Station (44°33’N, 76°19’W). We captured adult females and males using mist nests and traps placed around Nyjer seed feeders. We used only adult birds (ASY) in the study; sex and age‐class were determined by plumage (Pyle 1997). Birds were captured and sampled for T and bill color prior to clutch initiation, from 11 June to 3 July 2011. Bill coloration at this time of year has already undergone its seasonal change from grey to orange, and the orange color has reached its peak extent across the bill.
Upon capture, we collected basic morphometrics and objectively measured bill color. All measures of bill color were taken within 1 h of capture because bill color can change rapidly (Rosenthal et al. 2012). Bill color was measured with an Ocean Optics USB2000 + spectrometer and PX‐2 pulsed xenon lamp. The probe was mounted in a holder that minimized ambient light and positioned the tip of the probe approximately 7 mm from the surface of the bill. The probe was held 90° to the bill surface, and measured an area approximately 2 mm in diameter. Measures of reflectance were taken at five different, haphazardly chosen locations on left side of the upper mandible. Reflectance (R ) was quantified as the proportion of light reflected off the bill, compared to our Spectralon white standard. The white standard was protected in a housing to prevent the transfer of oil and dirt from the bill to the standard. The spectrometer was calibrated against this standard before measuring the bill color of each bird. We created a mean reflectance curve (from the five measurements) and calculated mean brightness (i.e. luminance; mean R from 320 to 700 nm, hereafter ‘brightness’), hue (wavelength where R = [R max+R min]/2), and yellow‐orange saturation ([sum of R from 550 to 625 nm]/mean brightness) using RCLR ver. 0.9.33 (Montgomerie 2010); see Table 3.2 in Montgomerie (2006) for further details.