Bird beaks are a textbook example of adaptations to feeding strategies, but there’s one major exception: birds of prey. While Darwin’s finches display a dazzling array of beak shapes and sizes – well suited for the seeds, nectar, or insects they eat – the beaks of eagles, ospreys, and vultures are all more or less the same shape. That’s because natural selection can’t always act on just beaks alone; it must contend with genetics and evolutionary history. According to new findings published in Proceedings of the National Academy of Sciences this week, beak and skull shapes in raptors are strongly coupled, and they’re both controlled by size.
Raptors are found in every habitat and on every continent except Antarctica. They range in size between 40 and 12,500 grams (1.41 and 440 ounces), and whether they eat rodents, carcasses, snakes, or fish, most raptors have strong, hook-shaped beaks. "Basically, if you're a bird of prey and you're small, you look like a tiny falcon, and if you're a bird of prey and you're large, your skull looks like a vulture," Jesús Marugán-Lobón of Universidad Autónoma de Madrid explained in a statement.
Marugán-Lobón and a team led by University of Bristol’s Jen Bright conducted morphometric analyses using 62 landmarks on the skulls of 147 species of raptors that hunt prey in the day. Raptors sit at the base of the landbird family tree, and there are five families of diurnal birds of prey: secretary birds, ospreys, falcons and caracaras, New World vultures, and the large family Accipitridae, which includes hawks, eagles, kites, harriers, buzzards, and Old World vultures.
The team found that raptor beaks can’t respond independently to natural selection: Their shapes are constrained to evolve in a particular way because the beak and skull are highly integrated structures that are strongly regulated by size. "In birds of prey such as eagles and falcons, the shapes of the skulls change in a predictable way as species increase or decrease in size," Bright explained. "The shape of the beak is linked to the shape of the skull, and these birds can't change one without changing the other."
Size and integration – and not dietary adaptation – account for about 80 percent of the variation in shapes that we see between different species. "Being able to break this constraint – letting the beak evolve independently from the braincase, may have been a key factor in enabling the rapid and explosive evolution of the thousands of species of songbirds," Bright added. While raptors use size as a mechanism to modify their feeding ecology, songbirds (like finches) were able to evolve new beak shapes altogether.
Image in the text: Red-tailed hawk. Donna Cole/Shutterstock