You're sitting in a park feeding pigeons when a car zooms by, causing them 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?
You have a free afternoon, so you decide to treat this like a scientist would. First, you repeat the experiment: yes, every time the birds take off they make that sound. Next, you 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 guessed that sound is made just by the action of pigeons 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 their 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.