Urban living suppresses carotenoid-based mouth color in mountain chickadee nestlings

The bright yellow flanges of nestlings in cavity-nesting birds can serve as visual signals, communicating both nestling presence and individual condition to parents. The development of this colourful signal depends on parental provisioning of dietary carotenoids—micronutrients responsible for the expression of integumentary colours. However, the availability of carotenoids can vary significantly among habitat types, influencing the expression of carotenoid-based traits. Some birds, such as mountain chickadees (Poecile gambeli), thrive in both urban and rural environments, despite differences in vegetation, food sources, and resource phenology between urban and rural habitats. Finding stronger carotenoid-based signals could imply habitat-specific differences in feeding behaviours or prey availability. Here, we tested whether the flange colour of mountain chickadee nestlings differed across habitats with variable amounts of anthropogenic disturbances, as well as synchrony between nest establishment and local leaf budburst. We used colour saturation values to compare relative carotenoid expression, analyzing colour patches from 120 nestlings from 22 nests in Kamloops, British Columbia, Canada. We found that six days post-hatching, nestlings living in habitats with greater anthropogenic disturbance exhibited reduced colour saturation, potentially indicating that nestlings in more highly urbanized locations experienced reduced carotenoid availability, or reallocated consumed carotenoids to mitigate physiological stress. The relative difference between the timing of nest establishment and local vegetation phenology associated with peak prey availability did not predict nestling carotenoid colour. This study provides insight into the influence of urban living on carotenoid-based ornamentation in altricial nestlings.