Iris colour of American kestrels varies with age, sex, and exposure to PCBs.

For most species of birds, little is known about the pattern and significance of intraspecific variation in iris colour. In early winter, captive American kestrels could be subjectively placed into at least two age categories: 0.5-yr-old birds had all-brown irides, whereas those of older birds were red-brown. As part of a toxicological study on kestrels, we quantified iris colour objectively using a digital camera to examine potential variation due to age, sex, and exposure to polychlorinated biphenyls (PCBs). Red, green, and blue values, plus an overall measure of colour using principal components analysis (PC1), were derived for breeding and nonbreeding kestrels 1.5-6.5 yr old (fed PCBs), plus offspring 0.5 yr old (exposed to PCBs only in ovo). Age category (0.5, 1.5, and 2.5+ yr) and PCB exposure consistently had an effect on colour, while sex was significant only for red and almost so for PC1. ANOVA with age as a covariate revealed that the amount of red continued to increase throughout life, but PCBs suppressed the development of that colour.