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Many songbird species, like happy wrens of western Mexico, duet with their mates. The two love birds sing song phrases back and forth with such precise coordination that, to the human ear, it sounds like one bird singing.
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Songbirds like robins or chickadees use alarm calls to warn about predators. These calls are often so specialized that they tell other birds what kind of predator is around and the best action to take (like flight or fight).
Some birds, like the broadbill in East Africa, can “sing” with their feathers. The microscopic structures of their feathers allow the birds to beat their wings in a specific way to create a trilling noise.