Songs are longer and more complex and are associated with territory[1] and courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact.

[2] Other authorities such as Howell and Webb (1995) make the distinction based on function, so that short vocalizations, such as those of pigeons, and even non-vocal sounds, such as the drumming of woodpeckers and the "winnowing" of snipes' wings in display flight, are considered songs.

The high frequency of female vocalisations in the tropics, Australia and Southern Africa may also relate to very low mortality rates producing much stronger pair-bonding and territoriality.

These calls are characterized by wide frequency spectra, sharp onset and termination, and repetitiveness that are common across species and are believed to be helpful to other potential "mobbers" by being easy to locate.

The social communication by vocalization provides a shortcut to locating high quality habitats and saves the trouble of directly assessing various vegetation structures.

The acoustic adaptation hypothesis predicts that narrow bandwidths, low frequencies, and long elements and inter-element intervals should be found in habitats with complex vegetation structures (which would absorb and muffle sounds), while high frequencies, broad bandwidth, high-frequency modulations (trills), and short elements and inter-elements may be expected in open habitats, without obstructive vegetation.

[58][59] Traffic noise was found to decrease reproductive success in the great tit (Parus major) due to the overlap in acoustic frequency.

[71] Both neural pathways in the song system begin at the level of HVC, which projects information both to the RA (premotor nucleus) and to Area X of the anterior forebrain.

[75] Brain structures involved in both pathways show sexual dimorphism in many bird species, usually causing males and females to sing differently.

Female zebra finches treated with estradiol after hatching followed by testosterone or dihydrotestosterone (DHT) treatment in adulthood will develop an RA and HVC similar in size to males and will also display male-like singing behavior.

For example, male zebra finches castrated or given sex steroid inhibitors as hatchlings still develop normal masculine singing behavior.

[82] Both the European starling (Sturnus vulgaris) and house sparrow (Passer domesticus) have demonstrated changes in song nuclei correlated with differing exposures to darkness and secretions of melatonin.

[83][84] This suggests that melatonin might play a role in the seasonal changes of singing behavior in songbirds that live in areas where the amount of daylight varies significantly throughout the year.

[85][86] The gene FOXP2, defects of which affect both speech production and comprehension of language in humans, becomes highly expressed in Area X during periods of vocal plasticity in both juvenile zebra finches and adult canaries.

[91] During the sensorimotor learning phase, song production begins with highly variable sub-vocalizations called "sub-song", which is akin to babbling in human infants.

Prather, et al. found that during the short period of time before and after the bird sings, his HVCX neurons become insensitive to auditory input.

[106] Specifically regarding birds, it is possible that the mirror neuron system serves as a general mechanism underlying vocal learning, but further research is needed.

In addition to the implications for song learning, the mirror neuron system could also play a role in territorial behaviors such as song-type matching and countersinging.

[110] The learned nature of bird song as well as evidence of "dialect"-like local variations have support theories about the existence of avian culture.

A recent study has shown that a dopamine circuit in zebra finches may promote social learning of bird song from tutors.

[116] Various studies have shown that adult birds that underwent stress during critical developmental periods produce less complex songs and have smaller HVC brain regions.

[119] Further investigation showed that male song sparrows with larger vocal repertoires required less time to solve detour-reaching cognitive tasks.

[120] Some have proposed that bird song (among other sexually selected traits such as flashy coloring, body symmetry, and elaborate courtship) allow female songbirds to quickly assess the cognitive skills and development of multiple males.

[123] Musical notation to depict bird sound began with Athanasius Kircher in his Musurgia universalis (1650) but more careful use was attempted with enhancements in the twentieth century by the Germans Alwin Voigt, Cornel Schmitt, and Hans Stadler.

[124][125][126] Kay Electric Company, started by former Bell Labs engineers Harry Foster and Elmo Crump, made a device that was marketed as the "Sona-Graph" in 1948.

For instance, some have argued that in order for a communication system to count as a language it must be "combinatorial",[140] having an open-ended set of grammar-compliant sentences made from a finite vocabulary.

Research on parrots by Irene Pepperberg is claimed to demonstrate the innate ability for grammatical structures, including the existence of concepts such as nouns, adjectives and verbs.

[147] Other notable birdsong recordists include Eric Simms, Chris Watson, Boris Veprintsev (Soviet Union),[148] and, in France, Claude Chappuis,[149] Jean-Claude Roché, François Charron and Fernand Deroussen.

[150][151][152][153] Authors including Rothenberg have claimed that birds sing on traditional scales as used in human music,[154][155][156] but at least one songbird does not choose notes in this way.

[158] Hollis Taylor's in-depth analysis of pied butcherbird vocalizations provides a detailed rebuttal to objections of birdsong being judged as music.