The song of birds can be soothing and inspiring, but birds sing for more than just the beauty of it. Birds use song, call notes and behavior to communicate with each other. Birds use sound and action to scare off predators or warn other birds about danger, to attract a mate or to defend one's territory.
Not all birds sing, but those that do are in a class of birds known as passerines, or perching birds. (The term "passerine" is sometimes incorrectly used as a synonym for "songbird," but this is inaccurate, since passerine status is defined by the structure of the bird's feet. All songbirds are passerines, but not all passerines are songbirds.) Many familiar backyard birds are songbirds, including sparrows, wrens, warblers and thrushes. The males of the species often sing more than the females. The males sing to announce their presence and to let females know that they are available for mating. They also sing to defend the territory in which they mate, nest or feed. Females do not sing as frequently as the males. A song is often a multi-noted phrase that is repeated over and over. Some species only have one song in their repertoire, while other species may have several. Some birds, such as starlings, will mimic the songs of other species of birds, and they may be able to produce dozens of different songs.
A more common form of communication among birds are call notes. Most birds communicate aurally, although some are more vocal than others, and each species of bird has a variety of call notes to convey different messages. Birds use call notes to alert other birds of danger, and some species may have different call notes for different threats (for example, they may have one note to sound the alarm for an airborne predator like a hawk or owl and another note for a land predator like a cat). Birds also use call notes to locate their mate or offspring or to communicate with other birds in their flock while they are flying. In smaller birds, call notes often sound like a chip, chirp or peep, and in larger birds the call notes may sound like a screech, caw or click.
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Birds also communicate with their behavior. In many bird species, the male will dance, strut or put on some other performance to attract a female. Some birds, such as killdeer, fake an injury to lure predators away from their nests. Many other birds behave aggressively if their nest or territory is threatened and may attack the interlopers, even if they are much larger than the birds are.