Why some birds lose the ability to fly

More than 99% of birds can fly, but some species have lost this ability over time. Penguins, ostriches, and kiwis evolved into flightless forms, adapting to their specific environments.

A recent study published in the journal Evolution examines how birds transition from flight to flightlessness.

The researchers compared the feathers and bodies of flightless birds with their closest flying relatives, identifying which traits change first.

These findings provide insights into evolutionary processes and help determine whether extinct birds were flightless.

Why some birds stop flying

All birds that are flightless today descended from ancestors that could fly.

“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,” noted Evan Saitta, a research associate at the Field Museum in Chicago and lead author of the study.

Birds often lose the ability to fly when they settle in predator-free islands. Without evolutionary pressure to escape threats, they gradually adapt to a ground-based lifestyle. Another common reason for flight loss is a shift to semi-aquatic living.

Penguins, for instance, “fly” underwater using their wings, leading to significant changes in their feathers and skeletons.

Flight loss in modern birds

Saitta, primarily a paleontologist, took advantage of the Field Museum’s vast bird collections.

“I suddenly had access to all these modern birds, and it made me wonder, ‘What happens when a bird loses the ability to fly?’”

Measuring various feather characteristics, Saitta compared preserved skins of 30 flightless species and their closest flying relatives. He also examined more distant species to expand the dataset.

Previous studies show when different flightless birds diverged from their flying ancestors. Ostriches lost flight much earlier than the Fuegian steamer duck. Their feathers reflect this difference.

“Ostriches have been flightless for so long that their feathers are no longer optimized for being aerodynamic,” said Saitta. Their long, shaggy feathers serve different purposes, such as insulation and display.

In contrast, the Fuegian steamer duck, which lost flight more recently, retains feathers similar to those of flying ducks.

Feather evolution after flight loss

Saitta was surprised at how long flightless birds retained flight-adapted feathers. Logically, one would expect unnecessary feather traits to change quickly. However, his postdoctoral advisor Peter Makovicky offered another explanation.

“Pete pointed out that when trying to understand why a modern bird looks the way it does, you can’t just think about natural selection or relaxation thereof. You have to also consider developmental constraints,” said Saitta.

Feathers develop in a strict sequence that is difficult to alter. When birds lose flight, their feather features disappear in the reverse order they evolved.

Feather evolution follows a predictable pattern. Flight feathers first evolved in dinosaurs and became increasingly complex. When a bird loses flight, these adaptations vanish in reverse order.

Some recent features, like asymmetric flight feathers, disappear quickly. However, fundamental feather structures take much longer to change.

Skeleton changes occur first

Saitta and his team also found that certain body features change more quickly than feathers.

“The first things 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,” explained Saitta.

Skeleton changes occur before feather modifications because bones require more energy to grow. Evolution prioritizes reducing these costs first.

“Let’s say a bird species lands on an island where they are able to safely live on the ground and don’t need to fly anymore. The first things to go are going to be these big, expensive bones and muscles, but feathers are cheap, so there’s less active selection to change them,” said Saitta.

Maintaining a flight-ready skeleton is a larger evolutionary burden than retaining unnecessary feathers.

Fossilized flightless birds

This study aids paleontologists who study fossilized birds and feathered dinosaurs.

“Flight didn’t evolve overnight, and flight, or at least gliding, was possibly lost many times in extinct species, just as in surviving bird lineages. Our paper helps show the order in which birds’ bodies reflect those changes,” said Saitta.

Fossilized feathers alone may not indicate whether an ancient bird could fly. Instead, researchers should first examine body mass and wing proportions.

These traits change before feathers do. If skeletal features suggest flight loss, feather asymmetry can provide further confirmation.

Past research on flight loss

Saitta’s study aligns with earlier research on feather changes after flight loss.

“The good news is that because I came at this question from a different angle, we got results that are very consistent with a lot of the previous research, but I think maybe a little bit broader than if I had approached the question with a more specific focus,” said Saitta.

By analyzing both skeletal and feather changes, this research offers a more complete understanding of how and why birds lose the ability to fly. It also provides valuable tools for identifying flightlessness in extinct species, adding another layer to the study of avian evolution.

The study is published in the journal Evolution.

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