A new report published in American Naturalist by a pair of ecologists, W. Alice Boyle and Courtney J. Conway, at the University of Arizona, in Tucson, has determined that the primary pressure prompting short-distance bird migrations comes from seasonal food scarcity, not their amount of eating or living in non-forested environments, as was previously thought. “It’s not just whether they eat insects, fruit or nectar, or where they eat them; it matters how reliable that food source is from day-to-day,” says Boyle.

A universal assumption has been that short-distance migration is an evolutionary steppingstone to longer trips. The team’s work contradicts that idea by showing that the two are inherently different. They also found that species that forage in flocks are less likely to migrate. “If a bird is faced with food scarcity, is has two options,” Boyle notes. “It can either forage with other birds or migrate.”