Loss of flight in waterfowl

What is it about?

Birds are well known for their ability to fly. However, not all birds fly well, and reductions in flight capacity — sometimes to the point of flightlessness — are more common than might be expected. In general, we know that flightless birds have a unique anatomy: compared to flight-capable relatives, they often have a smaller, “underdeveloped” flight apparatus, larger, “overdeveloped” legs, and sometimes, larger bodies. But how do these anatomical changes occur, and how do birds locomote as their wings are reduced? The answers to these questions are unknown. Here, we experimentally documented developmental mechanisms behind a transition toward flightlessness in a unique group of ducks: flight-capable Mallards, flightless Indian Runner Ducks (bred from Mallards), and their experimentally produced hybrid offspring. We found that Runner Ducks share many features with wild flightless birds, and that Hybrids are intermediate. Surprisingly, however, wings become relatively smaller due to increases in body size rather than reductions in wing area or muscle, which are conserved. Legs, in contrast, are labile: Runner Ducks invest proportionally less in their leg muscles as juveniles but more as adults than Mallards. This contrast between wing conservation and body and leg growth suggests that for at least some flight losses, bodies and legs change before wings, which get proportionally smaller before being reduced. Given that immature birds with incipient flight and adult birds with reduced flight both engaged their wings and legs at the same time, wing-leg coordination may facilitate shifts along a flightless-to-flying spectrum, exemplifying how specialized structures can be gained or lost.

Featured Image

Photo by Erwin Bosman on Unsplash

Photo by Erwin Bosman on Unsplash

Why is it important?

Most birds are specialized for flight. Yet flight is often reduced or lost, and how this occurs is unknown because most losses occurred long ago, so stages of flight loss have never been observed. In this manuscript, we experimentally documented developmental mechanisms behind a transition toward flightlessness in a unique group of ducks. Although flightless birds often have small, seemingly “useless” wings, our results suggest that small wings may initially result from increases in body size rather than reductions in wing size, and that birds incapable of sustained flight may still engage their wings and legs cooperatively. Such wing-leg coordination is also important to developing birds and shows how animals can use transitional behaviors as they gain or lose specialized structures.

Perspectives