They were able to determine which features change first when birds evolve to be flightless, versus which traits take more time for evolution to alter. These findings help shed light on the evolution of complex traits that lose their original function, and could even help reveal which fossil birds were flightless.

All of the flightless birds alive today evolved from ancestors who could fly and later lost that ability. "Going from something that can't fly to flying is quite the engineering challenge, but going from something that can fly to not flying is rather easy," says Evan Saitta, a research associate at the Field Museum in Chicago and lead author of the paper.

In general, there are two common reasons why birds evolve flightlessness. When birds land on an island where there aren't predators (including mammals) that would hunt them or steal their eggs, they sometimes settle there and gradually adapt to living on the ground.

Since they don't experience evolutionary pressure to stay in flying form, they gradually lose some of the features of their skeletons and feathers that help them fly. Meanwhile, some birds' bodies change when they evolve semi-aquatic lifestyles. Penguins, for instance, can't fly, but they swim in a way that's akin to "flying underwater." Their feathers and skeletons have changed accordingly.