How do falcons manage to catch prey midflight? Some animals, such as bees and tiger beetles, fly directly at their game. Others, like dogs and some fishes, pursue at an intercept angle based on the fleeing creature’s trajectory. Until now, however, falcons’ aerial pursuits have remained a mystery; because they take place across large distances and at high speeds, data collection on their techniques has been difficult.
Suzanne Amador Kane, of Haverford College, decided to get a bird’s-eye view. She and Marjon Zamani teamed up with professional falconers to mount small video cameras on the heads and backs of falcons. They then analyzed the videos, along with other similar footage that they found online.
Amador Kane suspected that the falcons did not fly directly toward their prey, a technique called classical pursuit, because that method is “very inefficient.” She adds, “What you’d really like to do is you’d like to fly and intercept your prey in the least amount of time.”
That suspected strategy turned out to be effective in a surprisingly clever way, however. The results of the analyses by Amador Kane and Zamani, published January 15 in The Journal of Experimental Biology, show that instead of flying directly at their targets (in this case, crows), the falcons use a technique called motion camouflage. While on an intercept course they also attempted to maintain a constant angle to the path of the crow. Using this approach, from the crow’s perspective the falcon appeared to stay in the same spot; although the hunter was closing in, it would not move across the crow’s visual field. As the subject changed direction or speed, the pursuers adjusted their flight to continuously remain at a fixed angle to the flight path.
The study also provides insights about falcon vision. Many birds of prey, including eagles and hawks, have two distinct spots on their retinas, called foveae, where their vision is sharpest. (Humans have only one such spot). The results of the study suggest that the angle the raptors maintained between themselves and their quarry corresponded to the direction best suited for using one of these spots. That is, the falcons flew a trajectory that kept the image of their prey focused on the parts of their retinas where they could see their prey most clearly.
Although the location of foveae in the eyes of predatory birds can vary among species—and has not been specifically determined for the species of falcons used in this study—Amador Kane says that their results match up well with the likely location of the foveae based on their position in other birds of prey.
The study highlights an important facet of avian vision, says Graham Martin, emeritus professor at the University of Birmingham in England who was not involved in the study. Although humans see best the things that are directly in front of us, for for birds of prey, “the frontal stuff is not particularly important,” Martin says. “Things that are important to birds are picked up in the lateral visual field,” meaning off to the side. This is part of the reason birds of prey are often struck by windmills—they are evolved to look for prey off to the side or down below, but not in front. By the time they perceive a fast-turning windmill blade, it may be too late to avoid it.
Of course, for just as long as carnivores have been chasing down and eating other animals, those prey have been trying to avoid becoming dinner. Amador Kane observed distinct evasive maneuvers by the crows. She described some of these maneuvers to a former fighter pilot. He observed, “You know, your crows are doing exactly what I used to do.”