In a new study in the journal Evolution, researchers compared the feathers and bodies of different species of flightless birds and their closest relatives who can still fly. 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.

All of the flightless birds alive today evolved from ancestors who could fly and later lost that ability. In general, there are two common reasons why birds evolve flightlessness. When birds land on an island where there are no 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 do not 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, cannot fly, but they swim in a way that is akin to ‘flying underwater’. Their feathers and skeletons have changed accordingly.

Previous research has revealed how long ago different species of flightless birds branched off from their flying relatives. The ancestors of ostriches, for example, lost the ability to fly much longer ago than the ancestors of a flightless South American duck called the Fuegian steamer. Dr. Evan Saitta from the Field Museum in Chicago Saitta and the corresponding author of the paper found that these species’ feathers are very different. “Ostriches have been flightless for so long that their feathers are no longer optimized for being aerodynamic,” Dr. Saitta says in a press release. As a result, their feathers have become so long and shaggy that they’re sometimes used in feather dusters and boas. But even though Fuegian streamers can no longer fly, they lost this ability relatively recently, and their feathers remain similar to those of their flying cousins.

Dr. Saitta says he was surprised by how long it seemed to take flightless birds to lose the feather features that would have helped them fly. It didn’t seem to make sense why a flightless species would “waste”” energy growing a bunch of feathers optimized for an activity that it no longer did, or why feathers no longer required for flight wouldn’t be freed up to evolve into a wide variety of forms.

When bird embryos develop feathers, those feathers increase in complexity in the same general order that those feather features first evolved in dinosaurs. After losing the ability to fly, birds lose those feather features in the opposite order that they first evolved. Some more recently-evolved feather adaptations, like the asymmetry in the flight feathers that allows birds to fly, are easier to change, and thus disappear relatively quickly once birds no longer need to fly. But overall, the basic feather structure is like those load-bearing walls. It takes a lot of evolutionary time for the underlying development of a standard feather to be transformed into producing something like a plume-y ostrich feather.

Dr. Saitta and his colleagues also found that certain larger features changed relatively quickly once a lineage lost the ability to fly. “The first thing to change when birds lose flight, possibly even before the flight feathers become symmetrical, is the proportion of their wings and their tails. We therefore see skeletal changes and also a change in overall body mass,” he says.

The reason behind this may be the comparative ‘costs’ to grow these features. When animals develop, it takes a lot more energy to grow bones than it does to grow feathers — so evolution “prioritises” changing the skeleton before the majority of the feathers.