Contact calls are seemingly haphazard sounds made by many social animals (such as a chicken's cluck). Contact calls are very different from many other types of calls (for example, alarm calls), as contact calls are not a specific signal, designed to communicate some specific information. It is rather a mixture of various sounds, accompanying the group's everyday business (for example, foraging). Contact calls are used to maintain audio contact with the members of the group. Some social animal species communicate the signal of potential danger by stopping contact calls, without the use of alarm calls. Charles Darwin wrote about this in relation with wild horse and cattle.
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Humming as contact calls
Joseph Jordania suggested that human humming could have played a function of contact calls in early human ancestors. According to his suggestion, humans find it distressing to be in full silence (which is a sign of danger for them), and this is why humans who are alone sometimes hum, whistle, talk to themselves, or listen to TV or radio during other activities.
Birds
Birds use contact calls in flight to establish location and to keep aware of each other's presence while flying and feeding. For some species, this call consists of a short, high-pitched sound, recognized and duplicated exactly by mates. Some fowl, such as geese,"honk" while in migration to communicate location and proximity to others in their flock.
Warning
Some species of birds have alarm calls to specifically warn other individuals of predators. The black-capped chickadee warns its kind of the level of threat an approaching predator is by the number of "de"s heard. Its call, chick-a-de-de-de, might indicate more danger than chick-a-de-de. Some calls reveal more details about an approaching predator, indicated by the pitch or speed. When there is a threatening enemy in the air, such as a hawk or eagle, the Florida Scrub-Jay warns other jays to seek cover by using a thin, shrill-like call. Contrastingly, an approaching predatory feline provokes a low-pitched "scolding" sound and calls on fellow jays for help in scaring the intruder away.
Comfort
Parrots kept as pets demonstrate contact calls with their human owners. Parrots make their call to establish that the human is within earshot, and continue to make the call (sometimes growing louder into a scream) until acknowledged. The screaming develops in pet parrots, as well as wild flock, when the animal feels like its needs are not being met because the contact call is not being understood.
Deer
Deer communicate with contact calls, alarm calls and other calls to signal when they are ready to breed, to feed, to locate others in their group, after they have shed their velvet, to signal frustration, to summon nearby deer and to intimidate unwanted visitors.
Non-aggressive
Aggressive
Ring-tailed lemurs
A ring-tailed lemur's contact call is used heavily to communicate and is classified on the level of excitement the lemur achieves in a situation. When low to moderately excited, the lemur moans; a "meow" is heard when the excitement is raised to a moderate level or when a situation will bring group togetherness. The loudest call occurs when a lemur is separated from its troop, and is recognizable by a distinct "wail."