Interest in female ornamentation has burgeoned recently, and evidence suggests that carotenoid-based female coloration may function as a mate-choice signal. However, the possibility that females may signal status with coloration has been all but ignored. Bill coloration of female American goldfinches (Spinustristis) changes seasonally, from dull gray in winter to bright orange in the breeding season. We conducted a series of aviary experiments in the breeding season to examine the signaling role of female bill color during both intra- and intersexual contests as well as during male mate choice. We tested for status signaling by examining whether caged females and males avoided feeding adjacent to female taxidermic models as a function of the model's bill color, which was experimentally augmented or dulled. We tested for a mate signaling function by giving captive males a choice between 2 live females with experimentally altered bill colors. Females avoided feeding near model females with colorful bills, but males showed neither avoidance of nor preference for females with more colorful bills. These results indicate that the female's carotenoid-based bill coloration signals status during competitive interactions and suggest that female bill color does not function as a mate-choice signal. This represents the first experimental evidence that a carotenoid-based coloration of females functions to mediate contest competition over food.
The American goldfinch is a socially monogamous passerine with biparental care (McGraw and Middleton 2009). In southern Ontario, pairs begin forming in spring and first clutches are initiated in early July. A successful nesting attempt requires about 31 days from beginning of egg laying to nest departure, and the entire breeding season for a local population lasts just over 2 months. Because of nest predation, few pairs raise more than one brood per season. In addition to feeding nestlings, males invest heavily in feeding their mates during incubation and early brooding. Both sexes defend the immediate vicinity of the nest (Stokes 1950; Coutlee 1967; Middleton 1979). In captivity, both sexes defend access to food (Popp 1987). Goldfinches appear to be most aggressive during the breeding season, but are also agonistic in winter flocks, particularly when feeding (McGraw and Middleton 2009). Females appear to be more aggressive than males during the nesting period and are often observed chasing intruders from the nest site (Coutlee 1967). Both sexes perform stereotypic head-up and head-forward threat displays (Coutlee 1967), suggesting a possible signaling role of coloration on or near the head.
This study was carried out in southern Ontario, Canada, at the Queens University Biological Station (44°33′N, 76°19′W). From 13 to 29 July 2008, we tested intra- and intersexual dominance and male mate choice by presenting females and males to model females placed adjacent to bird feeders in an aviary. From 22 June to 18 July 2007, we tested male mate choice by presenting males with 2 live females in a large outdoor aviary.
In both years, we captured birds in mist nests placed around Nyjer Seed Feeders. Sex and age-class were determined by plumage (Pyle 1997). On capture, basic morphological features and body mass were measured, and birds were banded. T.G.M. measured the color of upper mandible and plumage (throat and breast) of all individuals with an Ocean Optics USB2000+ spectrometer and PX-2 pulsed xenon lamp (Ocean Optics Inc, Dunedin FL) with the probe both providing illumination and measuring reflectance at 90° to the feather or bill surface. The probe was mounted in a holder that minimized ambient light and held the tip of the probe approximately 7 mm from the surface. We quantified reflectance (R) as the proportion of light reflected off the measured substrate compared with a Spectralon white standard (Labsphere Inc, NH), at 1-nm intervals across the avian visual range (320–700 nm). The instrument was calibrated against this standard for each bird. In 2008, we calculated the mean reflectance of 5 measures for each body region; in 2007, we calculated the mean reflectance of 2 measures for bill and the mean of 5 measures for plumage patches. Measures of plumage and bill were taken at different, haphazardly chosen locations within each color patch. Using mean reflectance curves, we calculated mean luminance (“brightness”; mean R from 320 to 700 nm), hue (wavelength where R = [Rmax + Rmin]/2), and yellow chroma ([sum of R from 550 to 625 nm]/mean luminance) using CLR 1.05 (Montgomerie 2008); see table 3.2 in Montgomerie (2006) for further details.