In North America, male birds sing to defend territories and attract mates. Females of at least 40 species of birds sing at least occasionally, but in most species, males do all the singing. (In many species native to warmer climates, both males and females sing during courtship.) Both male and female birds also vocalize alarm calls, calls for their parents to feed them, calls to communicate with each other, or other sounds whose purpose is not known. Some species also have characteristic sounds that are not vocal, such as drumming by woodpeckers; many birds make clacking sounds with their beaks.
Birds use a variety of different calls for interacting with family members, alerting one another to the presence of predators, keeping in touch while flying or migrating, or announcing they have found food. Some, such as jays and domestic chickens, even use different calls to warn of predators on the ground and in the air. Mates can often match each other's calls to identify their partners, even at a distance or in a large flock. Whether birds can recognize the voices of their young depends on the species. Those, such as gulls, that nest in large colonies can generally recognize their own offspring, and will not admit other chicks to their nest, but in species such as barn swallows that become independent soon after fledging, the parents may never learn to recognize their own chicks' calls.
Fledgling birds begin singing sounds that are the equivalent of human infant babbling, and gradually refine them into the proper song structure for their species. Some species must learn their songs from older birds, while others have innate ability with no need to follow an example. Most birds stop once they've learned their own species' song, but many, such as the common Northern Mockingbird, continue on to learn the songs of other species and even to mimic other sounds in their environment, such as rusty hinges. The brown thrasher may have as many as 2,000 songs! (Occasionally, birds that are not mimics have been observed singing the songs of other species, but this may result in their failing to mate successfully with a female of their own species.) Like human language, some birdsongs have noticeable regional "accents."
Two of the birds that are common in New York City are excellent mimics. European Starlings, which are related to mynahs, can imitate other animals, vehicle alarms, and even human speech. (Listen to them here.) Northern Mockingbirds can have as many as 200 different songs in their repertoire. (Thomas Jefferson kept one as a pet in the White House. Hear their typical species song here.) The gray catbird, common in city parks, also can learn to mimic other bird and animal calls. Blue jays can imitate several kinds of hawks, although the reason for this behavior is unknown.
Most songbirds sing short songs—averaging only two to six seconds—but some species can sing as many as 36 notes a second! Birds may sing their songs thousands of times throughout the day—males of some species sing up to 70% of the time during mating season, and some birds are known for continuously repeating their song. The best time to hear birds sing is at dawn on a spring morning. Birds are quieter during nesting season, to protect their nests.
More information on birds and their songs can be found at the Cornell laboratory of Ornithology's website, All About Birds. You can share your observations of birds in the city at Celebrate Urban Birds.