The number and coloration of white flank spots predict the strength of a cutaneous immune response in female Diamond Firetails, Stagonopleura guttata

Colour variation in birds is often used to signal functional differences between individuals and sexes, though white plumage has usually been disregarded because white feathers were thought to be cheap to produce and hence unreliable signals. Here, we provide evidence for sex-specific morphological and colour differences in the strikingly patterned but apparently monomorphic Diamond Firetail (Stagonopleura guttata). We found that males had longer and darker tails, wider lores and a darker bill, while females had significantly more white flank spots. These gender-specific trait differences could be used to signal individual quality in sexual or social interactions, and hence we examined the correlation between these traits and the cell-mediated immune response (phytohaemagglutinin, PHA test). During both the breeding and non-breeding seasons, we found a positive correlation between PHA response and white flank spot number as well as their ultra-violet reflectance in females, and a negative correlation with white chroma and hue. In males, there was a positive correlation between PHA response and red rump reflectance (hue, red-chroma). There was no association between PHA response and either tail length or lore depth in either sex. These results add to a growing body of evidence that female spot number and white plumage reflectance signal quality.