When birds give up flight, what happens to their bodies—and their feathers? Paleontologist Evan Saitta has been exploring that very question, tracing the evolutionary reversals that occur when avian species abandon the skies. A specialist in feather evolution who began his career studying the origins of feathers in dinosaurs, Saitta brings a unique perspective to the puzzle of flight loss. Now at the Field Museum of Natural History, he draws on one of the world’s richest collections of bird specimens to compare modern flightless birds with their airborne relatives—and their ancient fossil cousins.

In this Q&A, Saitta explains how bird bodies adapt when wings become obsolete, what feather changes can reveal about long-extinct species, and why evolution sometimes works like remodeling a house.

What inspired you to study why birds lose the ability to fly? 

While I was in grad school at the University of Bristol, I studied primitive feathers in non-bird dinosaurs. I then accepted a job at the Field Museum in Chicago as a postdoc in 2018. The museum has one of the best collections of bird skins in the world. The question of how feathers change and simplify after flight loss in extant lineages of birds is an intriguing comparison to the changes in feathers as they first evolved from simpler to more complex structures in dinosaurs. My boss, Peter Makovicky, and I thought it was a perfect opportunity!

When birds stop flying, what are the first body changes that happen?

We found that large-scale, macroscopic changes occur more rapidly after flight loss than most microscopic changes to the feathers. In particular, birds tend to increase in body mass, while decreasing their relative wing and tail fan size. Therefore, it appears mostly that metabolically costly bone and muscle are the first to alter. As far as feathers go, the only feature that appears to perhaps change relatively quickly (evolutionarily speaking) is the symmetry of the vanes on the flight feathers, a feature that would otherwise confer aerodynamic capability. Feathers become more symmetric after flight loss.

Why do birds that recently lost flight still have feathers built for flying? 

It is probably due to a combination of two factors. Feathers are integumentary structures analogous to hair, they are keratinous and constantly replaced. As such, they do not confer a huge metabolic cost which might otherwise lead to selection on feathers to simplify or reduce. Additionally, there appear to be developmental constraints the make it difficult for feathers to evolve rapidly after flight loss. Feathers are complex branching structures, and the sequence of developmental events at the feather follicle may not be easily altered.

How does studying modern flightless birds help us understand ancient fossil birds and dinosaurs? 

Flight has been lost among extant lineages of birds multiple times. It makes sense, because it is energetically easier to not fly than it is to evolve flight! Surely then, we can predict that some extinct lineages of birds and non-bird dinosaurs must have reverted back to terrestrial lifestyles after evolving gliding or powered flight, if the environmental conditions were conducive to living on the ground. Our results can be used to judge whether a fossil is secondarily flightless or not. For example, we might expect shortened arms and increased body size, but with feathers that are relatively aerodynamically adapted, but perhaps fairly fairly in their vanes. 

What are some surprising things you learned about how bird feathers evolve over time?

One cool result is that changes in feathers that occur after flight is lost are the reverse pattern you generally see in both their initial evolution from dinosaurs to birds as well as their developmental sequence in a follicle. The last features of feathers to have evolved in dinosaurs and the last to develop at a living birds follicle are the first to change after flight is lost! Meanwhile, features that are early to develop and were early to initially evolve, such as the first order of branching from quill to barbs, can only be lost after millions of years of flightlessness, as in the quills on the arms of cassowary.

You compare a bird’s body changes to remodeling a house—can you explain that analogy? 

As explained in the answer 5 above, the early-appearing features of feathers are less prone to evolving and simplifying after flight loss. When a house is built, there are certain integral parts that are assembled first, such as the foundation and main frame/supports. If you were then to remodel the finished house, it is easiest to alter the features that were last to be added, such as decor and wallpaper or paint.

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