Plumage and eggshell colouration covary with the level of sex-specific parental contributions to nest building in birds

<p>Interspecific variation in sex-specific contributions to prenatal parental care, including avian nest building, remain poorly understood. Here, we examined whether the colouration of the parents and of their eggs covary with sex-specific nest building contributions in 521 species of Western Palearctic birds. Having colourful plumages and laying colourful eggs are both costly because of the deposition of pigments (in feathers and eggs) and/or forming costly nanostructural substrates (in feathers), and so it is expected that those costs are traded-off against the costs of nest building. We therefore tested two predictions of this trade-off hypothesis, namely that species in which females alone build nests (1) exhibit sexual plumage dichromatism in that females are less colourful than males and (2) lay less colourful eggs. Using a phylogenetically-informed comparative approach, we found support for the first prediction so that species in which females build nests alone or together with males are more likely to be dichromatic in plumage. Meanwhile, regarding the second prediction, contrary to our expectation, we found that species in which females build nests alone or together with males are more likely to lay colourfully pigmented eggs relative to species in which only males build nests. This suggests that sex differences in plumage colouration and egg colouration covary in a complex manner with female pre-copulatory investment in reproduction at the interspecific level.</p>