Effect of feather abrasion on structural coloration in male eastern bluebirds Sialia sialis

We used observations of male eastern bluebirds captured twice within a breeding season to test whether changes in structural coloration are related to feather abrasion. Between first and second broods, the UV chroma and brightness of feathers decreased, while hue shifted towards longer wavelengths. Observed changes were greatest for feathers on the head, least for feathers on the rump, and intermediate for feathers on the back. For head feathers, we found a significant correlation between reduction in barb length and UV chroma. Plumage coloration at first capture was correlated with change in UV chroma such that the most ornamented males tended to lose more coloration. Moreover, the magnitude of UV color change was positively related to the number of days between color measurements.To test whether these changes were caused by abrasive properties of the nesting sites, we randomly increased or decreased the abrasiveness of nesting-box entrances by attaching sand paper or smooth plastic tape. The type of box entrance had no signicant effect on either coloration or barb length change. Our results suggest that feather abrasion is a factor in the seasonal color changes of bluebirds.