Evolution of songbird eggs colour

To understand why avian eggs are so variable in colour and patterning, we
investigated contemporary species that provide insights into the
evolutionary transitions that occurred during the early radiation of the
songbirds. We quantified egg colour and patterning from museum collections
of 269 species of Australian passerine and collated it to nest type data
(cup or dome nesting species). Using phylogenetically reconstructed trait
data, we showed that the ancestral passerine egg was likely to be white,
and to have been laid inside a domed nest. Egg colouration and nest type
were both phylogenetically clustered, and there was evidence of correlated
evolution between the two traits. As nests transitioned from domes to
cups, there was an increase in the range of egg colours observed,
presumably as a response to additional stressors. Finally, we found that
egg colour changes occurred more than twice as frequently in cup nesting
species than in dome nesting species. This suggests that cup-nesting
species may be using colour as an adaptive trait to compensate for the
loss of the protective nest roof.