Why do Pigeons make a whistling sound when they take off in flight?

Those aren't pigeons' voices but rather their wings you are hearing. We are hearing the sound of air passing through the feathers that are spread wide for acceleration, beating faster for lift, and spread wide for takeoff. Although we tend to associate this kind of high-frequency noise with hummingbirds, many birds produce similar tones, not unlike the sound of whistling through the branches of trees.
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