Only a few species of birds have no voice–storks, pelicans, and some vultures. Most birds produce some sort of vocal sound. The Passeriformes (perching birds,songbirds) are noted for their singing ability. Many birds are restricted to vocal sounds rather than songs or calls.
Call – a brief sound of simple acoustic structure- a peep, cheep, squawk, chatter, etc.
Song – a relatively long, often melodious, series of notes usually associated with some aspect of courtship.
The vocal organ of birds is composed of membranes located at the junction of the bronchi and called the syrinx. When a bird sings, air from the lungs is forced through the syrinx and air passing over the membranes causes a sound. Either one of the two membranes, or both, may be used in singing. Since two different sounds may be produced, the variation in both loudness and frequency is enormous.
The songbirds have the greatest number of muscles in the syrinx (4-9 pairs) while most other bird groups have only one pair. In general, the complexity of the syringial muscles is related to the complexity of the songs a bird can produce. But there are exceptions such as the American Crow, which has seven pairs of syringial muscles but a limited voice. Parrots on the other hand, can mimic the human voice and only have three pairs of syringial muscles.
Function of Song
Bird songs are basically related to reproductive activities in one way or another. Calls may be related to reproductive or self-maintenance activities. Some specific functions of song:
1. Proclaim sex/induce another bird to reveal its sex. There is a correlation between the complexity of the song and dull plumage and little or no sexual dimorphism. The duller the plumage and/or the more the sexes look alike, the more complex the song (e.g. Song Sparrows).
2. Attract a mate. True for many species.
3. Establish a territory. Often, males arrive on the breeding grounds before the females (e.g. Red-winged and Yellow-headed Blackbirds); singing establishes the territory and maintains it later.
4. Stimulate and synchronize courtship behavior. Indicates readiness to breed by both sexes.
5. Maintain pair bond. A female Song Sparrow will come off the nest and sing back to her mate.
6. Signal changes in domestic duties. May trade feeding or incubation duties, signaled by song.
7. Identify individual to young. Most precocial birds can recognize their own young – waterfowl and penguins, e.g. Experiments with Mallards indicate that communication between the female and young begins before hatching.
8. “Species” identification. A song or call may be given to identify a bird of the same species or population.
9. Hold flock together. Calls, usually, but often songs, are used to coordinate the movement of a flock; e.g. Pygmy Nuthatch, Mountain Chickadee.
10. Intimidate enemies. May be use to scare predators.
11. To perfect song through practice. Often, complex songs are learned through imitation and practice.
12. “Because they enjoy it”. Probably not
Basically a bird’s song is a very specific type of communication. If a predatory bird is perched conspicuously in a tree small birds will often make themselves known by a behavior called mobbing; they set up a chorus of calls around the predator to point it out and perhaps chase it away. But if the predator is flying overhead, the small birds race to the nearest bush or tree and utter their calls from there.
The calls may be very different. In the case of Chaffinches, they mob a predator with low pitched sounds described as ‘chink’ calls. But when they are in cover, the birds give a high-pitched thin note called a ‘seeet’ call which causes the birds to seek cover. The difference is that the chink note is easy to locate because of its low frequencies that differ in phase while the seeet call is composed of high frequencies which don’t allow it to be located because it is composed of high frequencies with little phase difference.
Many birds have warning calls that are species-specific. Crows give a warning call that will frighten away only other crows – this call has been recorded and used to scare other crows from cornfields.
Starlings roost in trees in cities and can be pests; they can also be driven away by species-specific calls – Robins and Grackles in the same flock are not affected. But in other cases, such as that of the Herring Gull, their alarm call will also scare away the Great Black-backed and the Laughing Gulls.
Parent birds can call to their young to make them ‘freeze’ in the presence of a predator, swim, peck at food, etc. Young birds can vocalize in such a way as to stimulate a parent to feed them, etc.
Song is typically the function of the male, but there are many exceptions. The female Mockingbird, Cardinal, and Black-headed Grosbeaks have songs as complex as the males’. In the phalaropes, where sexual dimorphism and courtship roles are reversed, the female only sings. And if females hold individual territories in the winter, they may sing then even though they don’t sing during the breeding season.
Each bird species exhibits a more or less characteristic song, but the song varies by age, sex, geographic location, and time of year.
Populations of the same species of birds having different songs are song races; each variation in a song is a dialect. The white-crowned Sparrow is well-known for having many song races and dialects. Geographic variation in song is very common. But within different populations of the same species, the song is more stereotyped (less variable) when there are other species present with similar songs. But when no other species with similar songs are present, there is more variation in the song within a population of one species. More variation is not only allowed, but it helps the recognition of single individuals.
Ecology of Song
Males of many bird species use a singing post to call their mate or establish territory from – tree. post, wire, etc. Other birds the live in grasslands like the Horned Lark or Bobolink have a flight song. Birds living in dense vegetation such as in rainforests or thick reed beds have loud voices since vegetation absorbs sound as well as obstructs vision. So birds have evolved calls and songs at least partly in response to the structure of the habitat in which the call is given. Weather also has an influence on bird song. Both cool and hot weather decrease the amount of singing, as do rain and wind.
A few species such as the Red-eyed Vireo sing more or less all day. But most birds sing more vigorously in the early morning and evening when there is less light. Some species sing at night, such as the Mockingbird and Nightingale. The amount of light rather than the time of day determines the beginning and end of singing. Cloudiness in the morning, for instance, will delay singing. Different bird species react to different amounts of sunlight. So some species in a particular area will begin to sing before others chime in (Dawn Chorus). The dawn chorus may begin at different times each day, depending on the amount of light, but the bird species will begin singing in the same order.
Most birds show a seasonal variation in some that is mainly correlated with breeding activities and hormone production. The richest, fullest song generally comes in the spring when birds are establishing territories and courting (unmated males sing more). After egg-laying commences, the birds sing less so as not to attract predators.
If a male bird renests or its mate is killed, it resumes full singing. In the fall after the breeding season, the bird stops unless it holds a winter territory.
For most species, hormones, stimulated by photoperiod, probably play a dominant role in determining the time of year a bird sings. The injection of male hormones into male birds in mid-winter will start them singing. (Used to do this to Canaries in pet shops – even females injected with hormones will start them singing.)
Inheritance and Learning of Song
Generally, calls are genetic while songs are partly inherited and partly learned. Many studies have been done of song development; the classic one is the one done in England on the Chaffinch. The full song of the male Chaffinch performs the function of keeping other males from its territory and attracting females. This song is described as: ‘chip-chip-chip, tell-tell-tell, cherry-erry-erry, tissy-chee-wee-oo’
When a young Chaffinch is taken from the nest and reared separately out of hearing of all Chaffinch song, its song development is greatly restricted. The bird eventually produces a song of about the normal length (2-3 seconds), but it fails to divide the first part of the song into phrases as a normally-reared Chaffinch does; or it does not end the song with the normal elaborate flourish. This simple, restricted song of the isolated bird represents the genetic basis of the Chaffinch’s song.
There is a short, critical six-week period at about eleven months of age during which the Chaffinch develops its final, refined song pattern. Once this critical learning period is over, the song is fixed for life, no matter how much a bird is exposed to other songs, it will continue to sing the final song pattern developed during this six weeks.
So what’s happening? In the wild, young Chaffinches learn some details of song from their parents or from other adults in the first few weeks of life. At this stage a young bird absorbs the general pattern of the song. But not until the critical learning period the following spring does the bird develop the fine details of the song. This is the time the young wild Chaffinch first sings in competition with other Chaffinches (for females, territory) and it learns the details of the song from its more mature neighbors. This, of course, leads to a bit of individual variation in song, although the general pattern is characteristic of the species. So song is an integration of both genetic and learned components and calls are entirely genetic.
A large number of birds exhibit varying degrees of vocal mimicry and imitate call notes of songs of other species. The Starling frequently mimics the Killdeer or nighthawk. Scrub and Blue Jays can imitate a Red-tailed Hawk.
The Mockingbird (and others in the family- Catbird, Brown Thrasher) are well known for imitating other bird’s calls. But there is no evidence to indicate that these calls are an attempt to communicate with the other species. They may, in fact, only be a human interpretation as birds hear songs much differently that we do. A Bullfinch in England was taught to whistle the English National Anthem. A lyrebird in Australia learned to whistle the noon whistle at a factory.
Parrots, mynahs, crows, and magpies apparently only mimic in captivity. And their imitation of a human voice is very different acoustically than the real human voice.
Evolution of Song
Birds evolved from voiceless reptiles. What is the evolutionary advantage of song for it to have developed to the extent it has?
1. Alarm notes to frighten predators.
2. Finding each other in dense vegetation.
3. As mobility increased with flight, the need for long-distance communication did also.
4. As a way of keeping a group together- for protection and during migration.
The Passerines have the most well-developed songs and calls, but other birds with less vocal abilities have developed other sounds. Kiwis stamp their feed when annoyed. Boat-billed Herons, Storks, and Albatrosses rattle or clap their bills. Woodpeckers drum. The Ruffed Grouse drums with its wings. The nighthawk and hummingbirds often make sounds with their wings or tails. A number of birds make whistling sounds as they fly through the air- may or may not serve a purpose.
More on songs from PBS.