Mountain Bluebird Colour and Temperature Data

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. Several
studies have examined how factors such age may influence structural
colour, however few studies have looked at how structural colour may be
influenced by environmental conditions such as variation in weather
conditions experienced during moult. In this study, we examined variation
in structural colour expression in relation to age as well as rainfall and
temperature during post-breeding moult for a population of Mountain
Bluebirds (Sialia currucoides) in Western Canada over nine breeding
seasons. Overall, we found structural colouration was explained by sex,
age, and weather patterns during moult. At a population level, tail and
rump feathers from males were more colourful (higher brightness and
chroma, hue values shifted more towards UV) than females, and adults were
more colourful than juvenals. Male and female rump feathers generally
became less colourful with age. Noise-receptor colour models revealed
colour differences were discernible among sexes, suggesting bluebird
colouration is an important sexual signal. Tail and rump plumage variation
was associated with weather during moulting periods, though the effects
were sex- and age-dependent. Female plumage was generally more colourful
following wetter and warmer early summers, while males were more colourful
following warmer late summers, and plumage of older birds was more
resilient to colour variation due to weather patterns. We suggest that
more rainfall may increase insect abundance and thus improve food intake
and overall condition of Mountain Bluebirds. This is one of the first
studies to examine how both age and weather conditions concurrently
influence the expression of structural colours in birds.