Effects of weather and age on feather colouration in mountain bluebirds

Birds exhibit a vast array of colours and ornaments and while much work has focused on understanding the function and evolution of carotenoid-based colours (red, orange, yellow), structural colouration (blue, green, purple, iridescent) can also play a key role in sexual signaling by acting as an indicator of individual quality. While several studies have examined how factors such as diet and age may influence structural colour, few studies have looked at how structural colour may be influenced by variation in weather conditions experienced during the annual moult. In this study, we examine variation in structural colour expression in relation to age as well as rainfall and temperature during moult for a population of Mountain Bluebirds (Sialia currucoides) breeding in Western Canada. We looked at feather colouration over a period of nine years by using reflectance spectrometry of rump and tail feathers from male and female Mountain Bluebirds and examined the relationship between feather colour and temperature and rainfall from the previous breeding season as well as age-related colour changes. At a population level, we found that feathers from males and females were more colourful (higher brightness and chroma, hue values shifted more towards UV) as birds aged from juveniles to adults. Within individuals, after they reached adulthood (second breeding season and beyond), male bluebird rump and tail feathers became less colourful.At a population level, male Mountain Bluebirds exhibited more colourful feathers following years with higher August rainfall, while female Mountain Bluebirds expressed more colourful tail feathers after years with higher July and August rainfall. However, these effects were dependent on age. Young female Mountain Bluebirds were most affected by weather conditions, exhibiting more colourful structural plumage after years with higher rainfall and higher August temperatures. We suggest thathigher rainfall may increase insect abundance and thus improve food intake and overall condition of Mountain Bluebirds, and we suggest that males and females may be affected differently due to differences in behavior during the breeding and post breeding periods. This is one of the first studies to show how both age and weather conditions may affect structural colours in birds.