Data from: The effects of life history and sexual selection on male and female plumage colouration

Classical sexual selection theory provides a well-supported conceptual
framework for understanding the evolution and signalling function of male
ornaments. It predicts that males obtain greater fitness benefits than
females through multiple mating because sperm are cheaper to produce than
eggs. Sexual selection should therefore lead to the evolution of
male-biased secondary sexual characters. However, females of many species
are also highly ornamented. The view that this is due to a correlated
genetic response to selection on males was widely accepted as an
explanation for female ornamentation for over 100 years and current
theoretical and empirical evidence suggests that genetic constraints can
limit sex-specific trait evolution. Alternatively, female ornamentation
can be the outcome of direct selection for signalling needs. Since few
studies have explored interspecific patterns of both male and female
elaboration, our understanding of the evolution of animal ornamentation
remains incomplete, especially over broad taxonomic scales. Here we use a
new method to quantify plumage colour of all ~6,000 species of passerine
birds to determine the main evolutionary drivers of ornamental colouration
in both sexes. We found that conspecific male and female colour
elaboration are strongly correlated, suggesting that evolutionary changes
in one sex are constrained by changes in the other sex. Both sexes are
more ornamented in larger species and in species living in tropical
environments. Ornamentation in females (but not males) is increased in
cooperative breeders—species in which female–female competition for
reproductive opportunities and other resources related to breeding may be
high. Finally, strong sexual selection on males has antagonistic effects,
causing an increase in male colouration but a considerably more pronounced
reduction in female ornamentation. Our results indicate that although
there may be genetic constraints to sexually independent colour evolution,
both female and male ornamentation are strongly and often differentially
related to morphological, social and life-history variables.