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Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs (often simply birdsong) are the sounds produced by birds that are melodious to the human ear. In ornithology and birding, songs (relatively complex vocalizations) are distinguished by function from calls (relatively simple vocalizations).

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The distinction between songs and calls is based upon complexity, length, and context. 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 song.^[3] Still, others require songs to have syllabic diversity and temporal regularity akin to the repetitive and transformative patterns that define music. It is generally agreed upon in birding and ornithology that certain sounds are considered songs. In contrast, others are classified as calls, and a good field guide will differentiate between the two.

Bird song is best developed in the order Passeriformes. Some groups are nearly voiceless, producing only percussive and rhythmic sounds, such as the storks, which clatter their bills. In some manakins (Pipridae), the males have evolved several mechanisms for producing mechanical sounds, including stridulation mechanisms similar to those found in some insects.^[6] The production of sounds by mechanical means, as opposed to the use of the syrinx, has been termed variously as instrumental music by Charles Darwin, mechanical sounds,^[7] and more recently sonation.^[8] The term sonate has been defined as the act of producing non-vocal sounds that are intentionally modulated communicative signals, produced using non-syringeal structures such as the bill, wings, tail, feet, and body feathers.^[8]

Song is usually delivered from prominent perches, although some species may sing when flying.

In extratropical Eurasia and the Americas almost all song is produced by male birds; however, in the tropics and to a greater extent the desert belts of Australia and Africa it is more typical for females to sing as much as males. These differences^[9][10] and are generally attributed to the much less regular and seasonal climate of Australian and African arid zones requiring that birds breed at any time when conditions are favourable, although they cannot breed in many years because food supply never increases above a minimal level.^[9] With aseasonal irregular breeding, both sexes must be brought into breeding condition and vocalization, especially duetting, serves this purpose. The high frequency of female vocalizations in the tropics, Australia, and Southern Africa may also relate to very low mortality rates, producing much stronger pair-bonding and territoriality.^[11]

The avian vocal organ is called the syrinx;^[12] it is a bony structure at the bottom of the trachea (unlike the larynx at the top of the mammalian trachea). The syrinx and sometimes a surrounding air sac resonate to sound waves that are made by membranes past which the bird forces air. The bird controls the pitch by changing the tension on the membranes and controls both pitch and volume by changing the force of exhalation. It can control the two sides of the trachea independently, which is how some species can produce two notes at once.

In February 2023, scientists reported that the possible sounds that ankylosaur dinosaurs may have made were bird-like vocalizations (in the above sense of production not at the larynx but then modified by it) based on a finding of a fossilized larynx from the ankylosaur Pinacosaurus grangeri.^[13][14]

One of the two main functions of bird song is mate attraction.^[16] Scientists hypothesize that bird song evolved through sexual selection, and experiments suggest that the quality of bird song may be a good indicator of fitness.^[17][18] Experiments also suggest that parasites and diseases may directly affect song characteristics such as song rate, which thereby act as reliable indicators of health.^[19][20] The song repertoire also appears to indicate fitness in some species.^[21][22] The ability of male birds to hold and advertise territories using song also demonstrates their fitness. Therefore, a female bird may select males based on the quality of their songs and the size of their song repertoire.

The second principal function of bird song is territory defense.^[16] Territorial birds will interact with each other using song to negotiate territory boundaries. Since song may be a reliable indicator of quality, individuals may be able to discern the quality of rivals and prevent an energetically costly fight.^[17] In birds with song repertoires, individuals may share the same song type and use these song types for more complex communication.^[23] Some birds will respond to a shared song type with a song-type match (i.e. with the same song type).^[24] This may be an aggressive signal; however, results are mixed.^[23] Birds may also interact using repertoire-matches, wherein a bird responds with a song type that is in its rival's repertoire but is not the song that it is currently singing.^[25] This may be a less aggressive act than song-type matching.^[25] Song complexity is also linked to male territorial defense, with more complex songs being perceived as a greater territorial threat.^[26]

Communication through bird calls can be between individuals of the same species or even across species. For example, the Japanese tit will respond to the recruitment call of the willow tit as long as it follows the Japanese tit alert call in the correct alert+recruitment order.^[27]

Individual birds may be sensitive enough to identify each other through their calls. Many birds that nest in colonies can locate their chicks using their calls.^[28] Calls are sometimes distinctive enough for individual identification even by human researchers in ecological studies.^[29]

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Over 400 bird species engage in duet calls.^[30] In some cases, the duets are so perfectly timed as to appear almost as one call. This kind of calling is termed antiphonal duetting.^[31] Such duetting is noted in a wide range of families including quails,^[32] bushshrikes,^[33] babblers such as the scimitar babblers, and some owls^[34] and parrots.^[35] In territorial songbirds, birds are more likely to countersing when they have been aroused by simulated intrusion into their territory.^[36] This implies a role in intraspecies aggressive competition towards joint resource defense.^[37] Duets are well known in cranes, but the sarus crane seems unique in infrequently also having three bonded adults defending one territory who perform "triets".^[38] Triets had a lower frequency relative to duets, but the functional value of this difference is not yet known.

A 2023 study found a correlation between the dawn chorus of male birds and the absence of females. The research was conducted in southern Germany, with male blue tits being the birds of interest. Researchers "found that the males sang at high rates while their female partners were still roosting in the nest box at dawn, and stopped singing as soon as the females left the nest box to join them". The males were also more likely to sing when the females entered the nests in the evening or even during the daytime. While this information is eye-opening, it still does not answer the question of why male birds sing more when females are absent.^[39]

Sometimes, songs vocalized in the post-breeding season act as a cue to conspecific eavesdroppers.^[40] In black-throated blue warblers, males that have bred and reproduced successfully sing to their offspring to influence their vocal development, while males that have failed to reproduce usually abandon the nests and stay silent. The post-breeding song therefore inadvertently informs the unsuccessful males of particular habitats that have a higher likelihood of reproductive success. The social communication by vocalization provides a shortcut to locating high quality habitats and saves the trouble of directly assessing various vegetation structures.

Birds communicate alarm through vocalizations and movements that are specific to the threat, and bird alarms can be understood by other animal species, including other birds, in order to identify and protect against the specific threat.^[41] Mobbing calls are used to recruit individuals in an area where an owl or other predator may be present. 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 alarm calls of most species, on the other hand, are characteristically high-pitched, making the caller difficult to locate.^[42]

Some birds are excellent vocal mimics. In some tropical species, mimics such as the drongos may have a role in the formation of mixed-species foraging flocks.^[43] Vocal mimicry can include conspecifics, other species or even man-made sounds. Many hypotheses have been made on the functions of vocal mimicry including suggestions that they may be involved in sexual selection by acting as an indicator of fitness, help brood parasites, or protect against predation, but strong support is lacking for any function.^[44] Many birds, especially those that nest in cavities, are known to produce a snakelike hissing sound that may help deter predators at close range.^[45]

Starlings imitate a variety of other avian species and have a repertoire of about 15–20 distinct imitations. They also imitate a few sounds other than those of wild birds. The calls of abundant species or calls that are simple in frequency structure and show little amplitude modulation are preferentially imitated. Dialects of mimicked sounds can be local.^[46]

Some cave-dwelling species, including the oilbird^[47] and swiftlets (Collocalia and Aerodramus species),^[48] use audible sound (with the majority of sonic location occurring between 2 and 5 kHz^[49]) to echolocate in the darkness of caves.

It has been proposed that birds show latitudinal variation in song complexity; however, there is no strong evidence that song complexity increases with latitude or migratory behaviour.^[50]

The range of frequencies at which birds call in an environment varies with the quality of habitat and the ambient sounds. 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.^[51][52][53]

Low frequency songs are optimal for obstructed, densely vegetated habitats because low frequency, slowly modulated song elements are less susceptible to signal degradation by means of reverberations off of sound-reflecting vegetation. High frequency calls with rapid modulations are optimal for open habitats because they degrade less across open space.^[54][55] The acoustic adaptation hypothesis also states that song characteristics may take advantage of beneficial acoustic properties of the environment. Narrow-frequency bandwidth notes are increased in volume and length by reverberations in densely vegetated habitats.^[56]

The hearing range of birds is from below 50 Hz (infrasound) to around 12 kHz, with maximum sensitivity between 1 and 5 kHz.^[22][57] The black jacobin is exceptional in producing sounds at about 11.8 kHz. It is not known if they can hear these sounds.^[58] The only bird known to make use of infrasound (at about 20 Hz) is the western capercaillie.^[59]

It has been hypothesized that the available frequency range is partitioned, and birds call so that overlap between different species in frequency and time is reduced. This idea has been termed the "acoustic niche".^[60] Birds sing louder and at a higher pitch in urban areas, where there is ambient low-frequency noise.^[61][62] Traffic noise was found to decrease reproductive success in the great tit (Parus major) due to the overlap in acoustic frequency.^[63] During the COVID-19 pandemic, reduced traffic noise led to birds in San Francisco singing 30% more softly.^[64] An increase in song volume restored fitness to birds in urban areas, as did higher frequency songs.^[65]

According to a study published in 2019, the white bellbird makes the loudest call ever recorded for birds, reaching a sound pressure level (SPL) of 125 dB.^[66][67] The record was previously held by the screaming piha with an SPL of 116 dB.^[68]

The acquisition and learning of bird song involves a group of distinct brain areas that are aligned in two connecting pathways:^[69]

• Anterior forebrain pathway (vocal learning): composed of Area X, which is a homologue to mammalian basal ganglia; the lateral part of the magnocellular nucleus of anterior nidopallium (LMAN), also considered a part of the avian basal ganglia; and the dorso-lateral division of the medial thalamus (DLM).
• Posterior descending pathway (vocal production): composed of HVC (proper name, although sometimes referred to as the high vocal center); the robust nucleus of the arcopallium (RA); and the tracheosyringeal part of the hypoglossal nucleus (nXIIts).^[70][71]

The posterior descending pathway (PDP) is required throughout a bird's life for normal song production, while the anterior forebrain pathway (AFP) is necessary for song learning, plasticity, and maintenance, but not for adult song production.^[72]

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. Information in the posterior descending pathway (also referred to as the vocal production or motor pathway) descends from HVC to RA, and then from RA to the tracheosyringeal part of the hypoglossal nerve (nXIIts), which then controls muscular contractions of the syrinx.^[69][73]

Information in the anterior forebrain pathway is projected from HVC to Area X (basal ganglia), then from Area X to the DLM (thalamus), and from DLM to LMAN, which then links the vocal learning and vocal production pathways through connections back to the RA. Some investigators have posited a model in which the connection between LMAN and RA carries an instructive signal based on evaluation of auditory feedback (comparing the bird's own song to the memorized song template), which adaptively alters the motor program for song output.^[72][74] The generation of this instructive signal could be facilitated by auditory neurons in Area X and LMAN that show selectivity for the temporal qualities of the bird's own song (BOS) and its tutor song, providing a platform for comparing the BOS and the memorized tutor song.^[74][75]

Models regarding the real-time error-correction interactions between the AFP and PDP will be considered in the future. Other current research has begun to explore the cellular mechanisms underlying HVC control of temporal patterns of song structure and RA control of syllable production.^[76] Brain structures involved in both pathways show sexual dimorphism in many bird species, usually causing males and females to sing differently. Some of the k