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From ostriches to kakapo parrots to flightless ducks, birds have lost the ability to fly many times. But some groups of birds end up grounded more often than others, and now it seems that those that moult all their flight feathers from both wings at once may be predisposed to evolving flightlessness.

Ryan Terrill at Occidental College in Los Angeles, California, was fascinated by moulting, the process by which birds shed and replace old feathers. As a graduate student at Louisiana State University in Baton Rouge, he was thinking about flightless grebes while conducting fieldwork in Bolivia and realised that every member of the waterbirds鈥� lineage 鈥� including the flying species 鈥� had something in common: all their flight feathers moulted at the same time.

He also noticed that this trait seemed especially prevalent among waterfowl like ducks, geese and rails. These are bird families that have produced a large proportion of the world鈥檚 flightless species.

鈥淚 realised they were all in those families that have simultaneous wing moult, which is really rare,鈥� says Terrill, adding that only 3 per cent of bird species shed all their wing feathers at the same time.

Curious if this meant they were somehow predisposed to becoming flightless, Terrill tested if the pattern held across thousands of species of birds. He developed evolutionary trees that included both living flightless and flying birds, and many flightless birds that became extinct in recent millennia. Much of the global flightless bird diversity has been lost to hunting by humans.

鈥淲e tend to think of [flightless birds] as kind of weird freaks, but the fossil evidence suggests that there used to be a lot of them,鈥� says Terrill. Before people settled on islands throughout the Pacific, the region was probably home to thousands of flightless rail species, he says.

Terrill used computer simulations to estimate how quickly and readily flightlessness evolved in bird groups with and without simultaneous wing moult.

He found that more than two-thirds of known flightless birds synchronise their wing moult. Across multiple versions of the evolutionary trees, Terrill found that these simultaneous wing moulters consistently lost flight faster than those with staggered feather replacement.

Birds like geese and rails already excel at finding food and escaping predators while temporarily flightless during their wing moult. So, these birds may be prone to becoming flightless, says Terrill. A bird that can survive brief periods without flight may be primed to lose it entirely, given the opportunity.

鈥淚t makes a lot of sense to me,鈥� says Natalie Wright at Kenyon College in Ohio, who wasn鈥檛 involved with the research. 鈥淭he things that are required of a bird if it鈥檚 going to be flightless for part of the year are probably really similar to what鈥檚 required for that bird to survive if it鈥檚 going to be flightless all the time.鈥�

Wright notes that a lot of the flying birds that experience simultaneous wing moult are using flight primarily during migration and for accessing new habitats.

鈥淎 lot of ducks use flight to move from one pond to another, but their flight is not their primary escape mode,鈥� explains Wright. 鈥淚t鈥檚 not how they get their food.鈥�

If such birds end up in an island habitat where long-distance movement is no longer necessary to find food, she says, flight becomes superfluous.