Colourful birds outshine brown species in cities, global study shows

A global study finds that bird species thriving in cities are more likely to display bright blue, grey or black plumage, while brown‑coloured species are the least successful. The research was led by Juan Diego Ibáñez‑Álamo of the University of Granada and Kaspar Delhey of the Max Planck Institute for Biological Intelligence. The team analysed colour data from almost all bird species worldwide against a reference database and published the results on 4 April in Ecology Letters. Their analysis shows that urban bird communities are richer in colour diversity once species richness is considered, disproving the urban colour homogenisation hypothesis. Birds with blue, grey or black feathers were more common in cities, whereas brown birds, typical of forest understory species, declined sharply. The study found no evidence that sexual dimorphism in colour is reduced in urban settings. Researchers suggest that lower predator pressure or other city factors may let birds be more conspicuous, but the exact cause remains unclear. The authors say cities act as evolutionary laboratories, and future work will test whether insects or mammals show similar colour patterns. Understanding these trends can help make cities more welcoming for diverse wildlife.

Context

Cities replace natural brown backgrounds with concrete and asphalt, reducing the advantage of cryptic brown plumage. Dark pigments like melanin may also help birds cope with pollutants. The findings add to growing evidence that urban environments shape animal evolution in subtle ways.