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

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.