Plumage colouration differs between offspring raised in natural cavities and nestboxes

Most of our knowledge on hole-nesting birds, including plumage colouration (an important component of visual signalling), comes from studies on populations breeding in human-provided nestboxes. However, as demonstrated in comparative studies, multiple parameters, such as cavity dimensions and microclimatic conditions, differ between natural and artificial cavities. Despite this, no study so far examined the impact of cavity type on plumage colouration to verify whether extrapolation of results from birds growing in nestboxes is justified. Here, we examined the impact of cavity type – natural cavities vs. nestboxes - on the carotenoid-based colouration of blue tit ( Cyanistes caeruleus ) and great tit ( Parus major ) nestlings. We found clear differences in plumage colouration depending on the type of cavity in which the birds developed. Our study adds to the growing body of evidence confirming that varying properties of natural cavities and nestboxes might influence nestling physiology, leading to phenotypic differences in the long-term.