Birds sing songs as well as give calls. Songs are constructed differently and serve a different purpose to calls. It is bird song which has captured the hearts of human beings.
Bird songs and singing is limited to only one order of birds, namely the Passeriformes. Even then not all passerines sing songs and probably less than half the birds on the planet are genuine songsters. The most famous song bird is the Nightingale (Luscinia megarhynchos). Writing in Rome before the time of Christ, Pliny said of the Nightingale, 'There is not a pipe or instrument in the world that can produce more music than this little bird does out of its throat'. Later Alfred Lord Tennyson declared, 'The music of the moon leaps in the plain eggs of the Nightingale'.
Learning songs and singing them is not easy for birds. The feats they produce for their small size are quite incredible. The European Wren (Troglodytes troglodytes) sings a song that contains 740 different notes per minute and which can be heard more than 500 metres away. Considering the comparative sizes of you, me and a wren, this is the equivalent of us singing a song that can be heard 4 or 5 miles away. Birds put an awful lot of energy into their singing. According to some estimates, singing in some birds at least is as energy expensive as flying. Birds do not just sing their songs now and then. Some birds sing practically non-stop through the breeding season. In Europe the Yellowhammer (Emberiza citrinella) sings its 'little bit of bread and no cheese' song over 3000 times a day. In North America however there lives a really hard working songster - the Red-eyed Vireo (Vireo olivaceus) has been recorded singing 'see me-hear me' 22197 times in a single day. It is nearly always the males who sing. Birds don't all sing all day, or all year. How birds choose when and where to sing is an important aspect of bird behaviour. Where you sing depends on who you are singing to and where you live. Male birds that are trying to attract a mate need to be sure that females can hear their songs as easily as possible. As well as being heard however, a male bird needs to stay alive. For this reason you will see that different birds adapt different strategies. Thus the European Robin (Erithacus rubecula) can usually be seen singing from a prominent post while Wrens give their renditions from well within the scrubbery. Sedge Warblers (Acrocephalus schoenobaenus) will climb slowly up a reed singing as they work their way up to the top before suddenly flitting down to nearer ground level again.
Bird song is sound and like all sounds it obeys some basic physical laws. Firstly, it attenuates or decreases in proportion to the square of the distance travelled from the source. Secondly, it can be absorbed and or reflected by physical objects, even such tenuous physical objects as the wind. Different wavelengths of sound (higher or lower sound) are absorbed and reflected to a lesser or greater degree by different objects. Scientists have done a lot of research into this sort of thing and they know that in a deciduous or tropical forest the best 'sound window' occurs between 1.5 kHz and 2.5 kHz. This means that sound in between these wavelengths survives and travels better than sound with higher or lower frequencies.
Another complication is that this only works if the bird is at least 1.5 metres above ground level. Surprisingly, in coniferous forest transmission of sounds between 1 and 3 kHz is enhance by the vegetation. Naturally enough, in an open grassland there is no real window but things like the sun, heating the ground and the air, change the way sound travels. However, height helps sound to travel further before it is all absorbed. So many grassland birds like Skylarks and Pipits sing on the wing, often making species 'song flights' to heights around 3-6 metres above ground level. One of the most amazing facts emerging from studies of the physical properties of bird song was the discovery that some birds like the Carolina Wren (Thryothorus ludovicianus) can judge how far away another male bird is singing not by the volume but by the amount of degradation that the song has undergone between its source and their hearing it.
A bird singing to keep other males out of his territory also needs to be heard. However he will often sing less intensely if this is his only reason for singing. Some birds like Reed and Marsh Warblers (Acrocephalus ssp.) sing less often, less intensely and less complicated songs after they have acquired a mate while Sedge Warblers stop singing altogether. The European Robin on the other hand sings all year round. We can hypothesise then that territory maintenance is more important in the non-migratory robin than in the migratory warblers. Research into warblers in Europe has shown that females prefer males with more complicated songs. Research also shows that in most birds the presence of a singing male is enough to maintain a territory. In other words the male's song is a bit like a sign saying 'This territory is occupied and the owner is home'. Other male birds then respect this. In most cases the resident bird wins any confrontation - possession is at least 9 tenths of the law in the world of birds.
The male sings for two reasons then; to proclaim to other males that this is his territory and to attract a mate. Often both these purposes are served by the same song. Not all birds sing as much as warblers and robins, however. The dawn chorus is a well-known phenomenon, however it has strong ecological reasons for its existence. The time just around dawn on the edge of a woodland is a good time to sing because the air is normally still and sound transmission is good. It is also a time when many of the daytime predators are not on the move yet. Thirdly, for insectivorous species food is not easy to find until later in the day when the air has warmed up. Fourthly, many female birds such as tits lay eggs in the morning and the best time to mate is in the hour before the eggs are laid. A male then is getting the best of several worlds if he sings well enough to attract a mate. The female can benefit as well because the quality of a male's song depends partially on the amount of feed he had the day before, so a male who sings well after a cold night with no food is likely to be good at finding food and therefore worth mating with.
Though some birds sing relatively simple songs others have more complicated ones involving many different phrases. Some birds such as Chaffinches (Fringilla coelebs) may have several, usually about 5, different songs they can sing. It seems that in species with complicated song, the females are turned on by the complexity of the song. Males with more complex songs therefore find mates earlier in the season. Getting the pair bond formed early in the season is important because the sooner the pair can start raising a brood of young the higher their chances of successfully rearing a second brood and thus increasing their reproductive success.
Information on this page was contributed by EarthLife.
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