You’re sitting in a park feeding pigeons when a car zooms by, causing them all to take off at once. The moment it happens you hear a kind of high-pitched whistling, resembling a startled exclamation. Surely these street-wise birds couldn’t be all that surprised by a car turning a corner. So why do they make that sound?
Before jumping to the answer, let’s ask how a scientist might proceed. First, repeat the experiment: yes, every time the birds take off they make that sound. Next, try having just one pigeon take off repeatedly, and do it enough times that the bird couldn’t be consistently surprised. Still you hear the noise. What’s your guess?
If you started to wonder whether the sound is made just by the action of birds taking flight, you went in the right direction. Those whistling noises are produced not by startled pigeon voices at all, but by air passing rapidly through feathers. When a pigeon first takes off, it stretches its wings wide and its individual feathers stand out. The edges of the feathers slicing through the air as it beats its wings make a rapid whistle, something like the dramatic whistle sword-fighters produce as they whip their rapiers back and forth.
Sometimes you can hear this in other large birds. The mourning dove, a close relative of the pigeon, has a particularly noisy take-off. Another good example are goldeneye ducks, which have such noisy wings they are known by hunters as “whistler” ducks. In smaller birds the effect is silent unless the wings are beating very rapidly, in which case the sound is closer to a low buzz — such as in the aptly-named hummingbird.