WHY ONLY MALES SING ?
The distinction between songs and calls is based upon complexity, length, and context. Songs are longer and more complex and are associated with courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact. Bird songs are best developed in the order Passeriformes. Most songs are emitted by males rather than females. Song is usually delivered from prominent perches although some species may sing while flying. Some are almost voiceless, producing only percussive and rhythmic sounds, such as the storks, which clatter their bills. In some manakins (Pipridae), males have evolved several mechanisms for mechanical sound production, including mechanisms for stridulation not unlike those foundn in some insects.
The production of sounds by mechanical means as opposed to the use of the syrinx has been termed variously instrumental music by Charles Darwin, mechanical sounds and more recently sonation. 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.
Embryos can hear, recognize and memorize calls
Each bird could recognize the calls of its own mate as distinct from those of others. Late embryos of many birds can hear, recognize and memorize calls from their parents and respond to them after hatching. Not only that they are able to communicate even with one another while still in the egg and maintain acoustic contact with the mother. The experiments have shown that after hatching the chicks responded to the calls to which they had been exposed while still in eggs. It proved that auditory signals could be learned even by the embryo before hatching.
In 1968 Kear, an English ornithologist studied in particular detail the vocalizations of unhatched mallard chicks. He found that at least three days before hatching, the chick embryos begin to inhale into their lungs the air enclosed in the egg’s shell. According to him this is the time they start communicating with one another by producing cheeping calls. The communication among the late embryos helps them all to hatch at the same time. In other words before coming out they deliberate and plan their hatching time. It has also been demonstrated in various experiments that the close proximity between the eggs is necessary so that the acoustic communication between the developing chicks can be facilitated.
It is believed that simultaneous hatching of all the chicks is of vital importance especially in terrestrial birds whose offspring start following their mothers immediately after coming out of the shell.
Another Englishman Collias showed that chicken eggs about to hatch, emit calls that can either induce their mother to come and warm them or provoke her to move off, so that they can cool down. This has also been proved that when danger threatens embryos stop producing sounds as soon as the mother emits even the faintest cluck of alarm. They even produce contented call when they have been turned by the mother.
Bird species use vocabulary of up to 15 calls for different purposes and the passerines (perching birds) are at the top of the list. Besides individual recognition, there are many other functions of the calls. However, there are different calls for different purposes; for instance, alarm call warns others of danger, roosting calls, social calls that keep the flock together, flight call, fight call, are associated with any particular event in a bird’s life, i.e. feeding calls, nest site calls, threat calls to keep the intruders away, aggressive call and so on.
Earlier it was believed that all the calls were inherited from the parents and were passed on from generation to generation unaltered, but recent researches have proved that this is not true. Many species including cardueline finch has learnt to imitate various calls and added in its repertoire. In most of the cases the capability to learn is maintained throughout the life.
Now the question arises as to why do birds take pains for learning new calls? There can be many reasons. One probability may be that the bond with new partner can be easily established in case the old one dies or moves away. The modifications affected in the calls through copying is so insignificant to human ears that it is almost impossible to make out the difference, but to the birds of the same species it is sufficient to discriminate one’s call from another’s.
Another interesting aspect of the bird call is that between different species it can sometimes convey information, which can be beneficial even to birds of other species. One notable example is the alarm call that is useful to all listeners around.
The study of “language” and communication in animals has now acquired a very important place in modern research into animal behavior, with this understanding songs and calls of birds has become the favorite subject of scientists. Quite a lot of work is being done to solve the mystery behind bird songs and efforts done in this direction have yielded good results and many interesting facts have come to light.
During the course of study it has been found that only a limited number of species utter musical sounds that can be justifiably called “songs”. Besides, it is also interesting to note that barring few exceptions in about 95% cases the male is the solo songster singing at certain times of the year. The great naturalist Charles Darwin says that the main purpose of the male’s song is to attract females, whereas Eliot Howard convinced many naturalists through his writing in Territory in Bird Life (1920) that the bird song is mainly meant for staking out and defending a territory.
Meanwhile, modern experts are of the opinion that basically male sings either to repel rival males or to attract females, but song may have other finer functions too. For example, it also prepares receptive females to the same state of readiness to mate as the rival male, so that mating is successful and fertilization takes place. In the case of warning other males and attracting females the same song can serve both the purposes, depending on the individual receiving it. A resident male may sing to drive away the intruding male trying to usurp his territory, yet he may use the same song to attract a passing female.
How do females avoid inbreeding?
If females mate with their own close relatives, they are less successful in breeding as compared to those that mate with distantly related males or those from outside their population. In such cases, how do females avoid inbreeding? Do they choose males singing songs different from those of their own fathers? A careful and detailed study shows that females are less likely than expected by chance, to mate with males that share their repertoire with that of the former’s father. Not only that they were also found turning down suitors whose songs were much different from their fathers’.
A study conducted on great tits revealed that females preferred to mate with males, whose song repertoires were slightly different from that of the females of their own fathers, but not very different. This shows that females, in choosing a male with which to mate, avoid close genetic relatives and incest, but at the same time also avoid out-breeding with genetically very distant males. Perhaps, avoiding such males mean, he may by adapted to different environmental conditions.
A song is often more complicated than a call, and experts associate this more complex signal with the transmission of a more complicated message. It has been observed that barring a few exceptions, birds do not sing when they are on the ground. Besides, some birds sing ‘prettier’ and elaborate songs than others. The reason is not known for sure, but if we accept Darwin’s argument that the song’s purpose is to attract females then, we also have to accept sexual selection, which would operate most strongly, especially when the competition for mates is very intensive. In such situation it would be logical to conclude that song elaboration may be an example of this kind of competition.
More than one song style
There are some males that have more than one song style. Experiments with canaries have proved that males with larger repertoire have larger song control centers in their brains and within a species they also have some kind of advantage when females choose their mates. Scientists believe that larger repertoire and singing more complex songs is, perhaps, an indication to the female that the male possesses good health and is full of vigour and vitality. Probably, it might also suggest something about the quality of the male’s breeding potential. Besides, in these birds males tend to learn more songs and enrich their repertoire as they grow older. Maybe the females find some relationship between the number of songs and the age of the male and choose the more experienced mature and successful ones.
Variations in bird’s language
Like humans, birds too have variations in their language. Those belonging to the same species with a wide geographical distribution (living in different areas) will sing songs that are not identical and the song of one will not be “understood” by the other. Just as local variations in human speech occur from place to place within a country sharing the same language, some birds too have small pockets of song variations, accurately described as local dialects. In this context an interesting fact that has come to light is that an English chaffinch cannot communicate with its Finnish cousins and faces difficulty in understanding what Central European chaffinch is trying to convey.
‘Local dialect’ factor
It is believed that the ‘local dialect’ factor also helps in choosing the mate. For instance, a bird adapted to polar region would best pair with another polar bird of the same species, in other words the individual speaking the same local dialect. Mating with a bird of same species, but belonging to desert area (with different local dialect) would dilute the gene pool and may possibly introduce inappropriate characteristics, and might make the chicks less likely to survive the harsh climate of the polar region.
Like calls, songs too are not uniformly innate feature of birds. They have to be “learnt”. Experiments conducted with numerous species have shown that those that are quite “musical” in the wild when reared in isolation have poor development of songs. It has been found that males sing abnormal songs, if deprived of an opportunity to hear adult song during the first nine months of life, but if played recorded tapes of (in the case of chaffinch) songs, they would learn to sing accurately as early as ten days after hatching.
In the wild, birds imitate the songs and sounds of not only other bird species, but also other animals and thus enrich their own repertoires; best examples of such birds are parrots, mynahs, starlings, ravens, some thrushes and magpies, etc.
Long bouts of singing and counter-singing
Another interesting and puzzling question is, why do birds indulge in long bouts of singing and counter-singing at dawn and dusk? No definite and clear-cut answer is available to this question. The information we have, today, is based on field observations, which shows that in the evening, when it is hard to find food, many birds get to seeking nesting space. In such situations, the territory-holders face maximum pressure from such birds. So to keep them away, they have to sing. Then, at dawn, when weather is fine and every living being feels a surge of fresh energy, the birds also might be getting in mood to sing and enjoy, more so, because at that time the transmission of sound is most effective in the absence of various disturbances, they find it as the is most appropriate time for declaring about their territorial rights
Some birds can sing in two quite distinct tunes at the same time. In birds, the syrinx (vocal apparatus) is at the bottom, where the two bronchi meet, while in humans. The larynx (equivalent to bird’s syrinx) is at the top of the trachea or windpipe. The syrinx sits astride the junction. Many birds have separate vibrating membranes or chords on the two arms of the syrinx with the result they possess the remarkable ability to produce two sounds at the same time.
The Reed warbler and brown thrasher can both sing two tunes at the same time. Different notes come from each half of the syrinx at exactly the same moment. But, the mystery that baffles more is, how the brown thrasher makes four sounds at the same point of time in its songs? Another mind-boggling fact is, as to how the north Australian Gouldian finch maintains a bagpipe-like drone over two independent songs sung at the same time?
The range of frequency over which birds produce songs spans at least seven octaves, from 80 to 11,000 cycles per seconds. When allied with such techniques as amplitude and frequency modulation and the spacing of notes, the information-carrying capacity of bird song is obviously very high.