Dark nests and egg colour in birds: a possible functional role of ultraviolet reflectance in egg detectability

Abstract

Due to the conspicuousness of ultraviolet colour in dark environments, natural selection might have selected for ultraviolet egg coloration because it would enhance egg detectability by parents in murky nests. Here we tested this hypothesis by using comparative and experimental approaches. First we studied variation in egg coloration of 98 species of European passerines measured using UV-visible reflectance spectrometry (300-700 nm) in relation to nesting habits. Analyses based on raw data and controlling for phylogenetic distances both at the species and the family level revealed that hole-nester species showed eggs with higher ultraviolet reflectance than those nesting in open habitats. The experimental approach consisted on the manipulation of ultraviolet reflectance of experimental eggs introduced outside the nestcup of the hole-nester spotless starling Sturnus unicolor and the study of retrieval of these eggs. Ultraviolet-reflecting eggs (“controls”) were more frequently retrieved to the nest cup than non-reflecting (“-UV”) eggs. These results were not due to “-UV” eggs being recognized by starlings as parasitic because when a parasitic egg is detected, starling removed it from the nest-box. Therefore, these results are consistent with the hypothesis that ultraviolet egg colours are designed to provide highly detectable targets for parent birds in dark nest environments.