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.

We quantified nest-building contributions, and the colouration of parent birds and their eggs of 521 bird species, included in the Birds of the Western Palearctic book series. Nest building contributions were classified as being ‘male’ if males build nests alone, ‘female’ if females build nests alone, ‘both’ if both parents contribute to nest building, and ‘neither’ if none of the parents contribute to nest building (e.g. seabirds). Sexual plumage dimorphism was classified as being either present or absent. Data on clutch sizes and body mass of males and females were extracted from the same source and were averaged per species. Eggshell colour was determined by reading species descriptions and by inspecting colour plates and was defined as being blue, brown, or white. We deemed blue or brown but not white (unpigmented) as pigmented by predominantly biliverdin or protoporphyrin IX, respectively.

Variable names and descriptions

Species - scientific name of bird species
Nest_builder - nest builder sex (both, male, female, neither)
Colour_dichromatism - sexual plumage dichromatism (yes/no)
Egg_colur - egg colouration (blue, brown, white)
Mean_BM - average adult body mass in grams
Clutch_size_mean - average clutch size