Not a great crooner Georgette Douwma/Getty

The call of a male peacock is no pleasure to listen to, but its splendid tail means it doesn’t matter. Now an analysis of more than 500 species shows that this is a common trade-off in the bird world: the best lookers aren’t the most talented singers, while the best vocalists aren’t as easy on the eye.

Sexual selection is an evolutionary process that shapes traits that animals use to attract mates, and birds are well known to resort to elaborate songs and flashy feathers in the name of reproduction.

To investigate which species use which traits, Christopher Cooney at the University of Oxford and his colleagues collected the songs of 518 species, and compared these with their feather colours. In particular, they looked at how much feathers differed between the males and females of each species – a sign that sexual selection has influenced their plumage.


Dull but delightful

They found that birds in which one sex has more showy plumage than the other tend to have less interesting, more monotonous songs. In species in which the males and females more closely resemble each other, the males sing longer songs over a larger range of musical notes.

The reason why bird evolution favours one trait over the other is unclear. It might be that birds living in dense forests with lower visibility rely more on their songs instead of colour to attract mates, but Cooney’s analysis didn’t find any relationship between sexually selected traits and habitat.

Instead, his team think that mate-attracting traits are costly to develop, so a species tends to evolve only one. Alternatively, once one attractive trait has begun to emerge, it may simply be pointless to develop a second.

Journal reference: Royal Society Proceedings B, DOI: 10.1098/rspb.2018.1557