The evolution of diversity in bird beaks is revealed in an analysis of more than 2,000 species published this week in Nature.

It is well known that natural selection drives the diversification of small groups of organisms to exploit available niches in their environment. The rate of evolution is rapid at first, and then slows as the niches become filled. However, because similar slowdowns have not been observed at larger taxonomic scales, it is unclear how ecologically relevant traits - such as bird beaks or bills - diversify across an entire class of organisms.

To address this paradox, Gavin Thomas and colleagues generated three-dimensional scans of bird bills from museum specimens representing more than 95% of living bird species and initiated a citizen science project called Mark My Bird to analyse bill diversity on a global scale. They find that bill shape expanded rapidly early on in the evolutionary history of today’s birds and slowed over time, while the rate of evolution continued to vary among individual lineages. The authors suggest that this finding supports the concept of mega-evolution, in which initial diversification of a trait is fine-tuned as organisms expand across the world and unpredictable events open up new ecological opportunities.

“Perhaps the most powerful message of the current study is that large-scale evolution is contingent on both history and catastrophe,” writes Bhart-Anjan Bhullar in an accompanying News & Views article.