Avian nuptial plumage coloration changes differently when exposed to urban and rural areas

Abstract

Abstract Avian plumage coloration could be altered by external factors like dust accumulation, but the effect of environmental conditions on such post-moult colour changes are poorly studied. Here, we investigated how exposure to the atmosphere in habitats of differing anthropogenic pressures modifies UV/blue structural plumage coloration — a sexually selected trait in eastern bluebirds Sialia sialis. We collected feather samples from live birds and distributed them in three types of habitats: (i) urban roads with low traffic, (ii) urban roads with high traffic, and (iii) rural roads with low traffic. After 4 weeks, the brightness of feathers decreased in all types of habitats-resulting in less-ornamented coloration. Changes in UV and blue chroma, however, varied with habitat type; chroma decreased in urban areas (becoming less ornamented), but increased in rural locations (becoming more ornamented). According to physiological models of avian colour vision, however, these changes in coloration tended to be below perceptual thresholds. Our results underscore the importance of how human activity can influence plumage colour of individual birds during the period between moult cycles which could have an impact on avian visual signalling. The study opens venues for further studies focusing on how the optical properties of specific airborne particles can influence coloration of wild-living animals. Significance statement Plumage coloration plays an important role in avian visual communication. In addition to moult-based change, coloration may change between moult cycles due to exposure to external factors like, for example, dust accumulation. Ecological factors modifying a passive plumage change, however, remains poorly studied. We collected samples of structurally coloured feather from a species wherein plumage colour is sexually selected and exposed them to outdoor conditions. We demonstrate that UV/blue chroma of coloration changes differently in urban and rural habitats. It increased in rural and decreased in urban areas. On the other hand, brightness decreased in both habitat types. Using physiological models of avian colour vision, however, we found that these colour changes were likely not perceptible to birds. Our study provides the first evidence that changes in coloration of fully grown feathers may be habitat dependent.