American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Simultaneous wing molt as a catalyst for the evolution of flightlessness in birds”

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Ryan S. Terrill (Dec 2020)

Why do some birds lose flight more readily than others? This research finds a link with rare form of wing feather molt

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Even the most complex and essential traits of animals can be lost under the right conditions. Eyesight, limbs, and flight have been lost repeatedly when the benefit no longer outweighs the cost of maintaining such capabilities. Yet some groups of animals seem to lose traits more readily than others. Why does trait loss differ among groups of animals even under seemingly identical ecological conditions? The answer may lie in features that predispose animals to lose such traits.

We’ve long known that certain habitats favor the loss of flight in birds. Flight is intertwined with birds’ lives, but many species of birds have lost flight, especially on islands and in aquatic environments. Even though loss of flight has evolved many times in birds, it seems to happen more often in some groups of birds than others, especially ducks, geese, grebes, and rails. This study builds off an observation that these birds all share a common feather replacement (molt) strategy that involves simultaneous loss of all flight feathers, resulting in a temporary flightless period. This temporary flightless period may force these birds to forage and escape predators without flying, thereby predisposing them to permanent flightlessness when conditions favor. If simultaneous wing molt does indeed predispose birds to loss of flight, then we should expect elevated rates in loss of flight in birds with this wing molt strategy. This study finds consistent support for this prediction across many possible evolutionary trees of birds. These results illustrate how some traits may preclude or catalyze the loss of other traits, as well as how molt strategies in birds may have long-term and profound consequences on their evolutionary trajectories.


Complex features such as vision, limbs, and flight have been lost by many groups of animals. Some groups of birds are more prone to loss of flight than others, but few studies have investigated possible reasons for this variation. I tested the hypothesis that a rare strategy of flight feather replacement is involved in rate variation in the evolution of flightlessness in birds. This strategy involves a simultaneous molt of the flight feathers of the wing, resulting in a temporary flightless condition during molt. I hypothesized that adaptations for this flightless period may serve as preadaptations for permanent flightlessness under conditions that favor permanent loss of flight. I found an elevated rate of loss of flight in lineages with simultaneous wing molt, when compared to loss of flight in lineages without simultaneous wing molt. This may indicate that birds with simultaneous molt are more prepared to adjust quickly to open niches that do not require flight, such as terrestrial niches on island habitats. These results illustrate how molt strategies can influence the long-term evolutionary trajectories of birds, and provides insight into how phenotypic precursors may act as a mechanism of rate variation in the loss of complex traits.