Aaron M Olsen
Bird beaks are often cited as a classic example of evolution by natural selection. Over generations, it is thought that beak shapes that are better suited to eating particular foods will outcompete less favourable beak shapes and be passed on to offspring, through a process known as adaptation. Over time this would cause birds’ beaks to closely match the foods they eat. This classic example of evolution matches our experience—we can immediately identify birds with cone-shaped beaks, like cardinals, as seed-eaters or birds with long, dagger-like beaks, such as herons, as fish-eaters.
However, the basic idea that we can predict a bird’s diet from the shape of its beak, or vice versa, has not actually been tested extensively in birds, owing in part to the challenge of collecting detailed, quantitative diet data from multiple bird species. Fortunately, we already know a lot about the foods eaten by waterfowl (e.g. ducks, geese, and swans) because of their economic and ecological importance. This study combines data compiled from over 150 waterfowl diet studies with 3D beak shape data measured from natural history specimens to ask how much of waterfowl beak shape variation can be explained by the foods they eat, why particular beak shapes match particular diets, and how waterfowl beak shapes have evolved over time.
This study finds that most of the variation in waterfowl beak shapes can be explained by diet and that what we think of as a “goose-like” beak (shorter, taller, and with a top edge that curves upward rather than downward) has likely evolved multiple times within waterfowl from a more “duck-like” beak. A duck-like beak is associated with a diet of aquatic seeds and small invertebrates while a goose-like beak is associated with a diet of leaves. This study also shows that the advantage of a goose-like beak for eating leaves can be explained by simple lever mechanics: a taller and shorter beak increases the strength of a goose’s bite, which helps it to eat tough plant parts.
Image caption: Black-bellied whistling duck feeding in Florida. Photo by Arlene Koziol.