Cooperation between species may be an overlooked factor in community organization.

Ecologists tend to assume that all of the species within a community are more different from one another than chance would predict, because competition precludes two species from sharing the same ecological niche. So mixed-species bird flocks are a puzzle: these groups, which contain birds of different species that all eat roughly the same food, seem to show species happily co-existing. According to an analysis of a global data set by Kartik Shanker at the Indian Institute of Science in Bangalore and his co-authors, the more similar two birds are by taxonomy, body size and foraging style, the more likely they are to be found together, especially where tropical mixed flocks are concerned.

Credit: RANGU NARAYAN

The team suggests that this happens because cooperative benefits — such as those provided by alarm calls or the discovery of food resources — are best obtained from the most similar individuals.

Am. Nat. 180, 777–790 (2012)