Do city and forest blue tit chicks respond differently to light at night?

Details

Urbanization drives variation in animal traits, including activity patterns, aggression and hormone levels. A challenge in urban evolutionary ecology is identifying the environmental drivers of these differences and asking whether individuals develop tolerance to urban factors that underlies adaptive responses and contributes to urban population evolution. Most evidence so far comes from correlational studies, with few field experiments manipulating a single urban-related factor.

Researchers experimentally tested whether urban and rural blue tit nestlings (Cyanistes caeruleus) differed in their glucocorticoid (CORT) response to artificial light at night (ALAN). ALAN has been suggested to alter CORT responses in several animal species, but no previous study had investigated whether the effect differs between urban and rural populations.

Immediately after hatching, urban and forest broods were either exposed to 2 lux of ALAN from an LED mounted inside the nestbox or received no treatment as a dark control. The experiment continued until the chicks fledged. At 13 days old, the researchers collected plasma to measure baseline CORT concentrations and feathers to provide an integrated measure of CORT during growth.

Forest chicks had higher plasma CORT (pCORT) concentrations than urban chicks, regardless of light exposure. Feather CORT responses to ALAN, however, differed by population. Urban chicks exposed to ALAN had higher feather CORT than urban dark controls, whereas forest chicks exposed to ALAN had lower feather CORT than forest dark controls. Higher pCORT was associated with lower fledging success regardless of population or treatment; feather CORT was positively associated with fledging success in ALAN-exposed broods but negatively associated in dark controls.

The researchers conclude that ALAN can help determine the glucocorticoid phenotype of wild animals and may contribute to phenotypic differences between urban and rural animals.