Reed College Canyon

Sexual Dichromatism and Condition Dependence in Ultraviolet Coloration of two Species of Jay (Corvidae)

author: Arthur Bass
advisor: Stephen Yezerinac
year: 2002

ABSTRACT: Bird plumage coloration is the result of reflection from pigmentation within feathers or the scattering of light by the microstructure of feathers (structural colors). Structural colors include blue, green, violet, and ultraviolet. Since humans cannot see in the ultraviolet range, it is possible that birds that appear monochromatic may be displaying variation in the UV range. Furthermore, these ultraviolet signals may depend upon the health of the bird at moult, making them indicators of quality. I captured Western Scrub Jays and Steller?s Jays and measured UV reflectance of various plumage regions with a UV/VIS spectrophotometer. In addition, spectral characteristics were compared to three indices of condition; feather growth rate, feather mite infestation, and weight relative to body size. I determined the sex of each bird from their DNA. Sexual dichromatism was evident for multiple spectral characteristics and plumage regions of both species. The rump region displayed the greatest amount of sexual dichromatism in both species. UV-chroma (proportion of UV reflectance to total spectrum) was the most common sexually dichromatic spectral characteristic. Body size was signficantly positively correlated and feather mite infestation was significantly negatively correlated to several spectral characteristics in Scrub Jays. These results demonstrate that sexual dichromatism is present in the UV range for both species. Furthermore, the expression of UV reflectance is a condition dependent trait in Scrub Jays. This study adds to a handful of recent studies showing UV signals may be common amongst structurally-colored birds.