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

Abstract

Owing to the conspicuousness of ultraviolet (UV) colour in dark environments, natural selection might have selected UV 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 (300-700nm) in relation to nesting habits. Analyses based on raw data and controlling for phylogenetic distances both at the species and the family levels revealed that hole-nester species produced eggs with higher UV reflectance than those nesting in open habitats. The experimental approach consisted of the manipulation of UV reflectance of the experimental eggs introduced outside the nest-cup of the hole-nester spotless starling Sturnus unicolor and the study of the 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, starlings removed it from the nest-box. Therefore, these results are consistent with the hypothesis that UV egg colours are designed to provide highly detectable targets for parent birds in dark nest environments.

Figure 1

Differences (mean±s.e.) in egg colouration ((a) brightness; (b) blue–red and (c) ultraviolet) in relation to nesting habits in western Palaearctic passerine birds. The numbers of species appear on error bars.

Figure 2

Relationship between mean blue–red and UV colour and proportion of hole-nesting species per family in Western Palaearctic passerine birds: (a,c) non-controlling and (b,d) controlling for the phylogenetic effects using independent contrast analysis.

Figure 3

Starling pairs retrieving eggs within the nest-cup in relation to a treatment affecting UV reflectance. Starlings can retrieve the control, the ‘–UV’ or the two kinds of experimental eggs (N=25 tested pairs with the two egg treatments).