In bird biology the word "song" means any sound delivered in the nesting period, usually by a male bird for the purpose of territorial defence or the attraction of a mate.
Song and songbirds
The purpose of song is self-advertisement, whether it is from a mockingbird amid the magnolia flowers, a European robin among the apple blossom, a tawny owl under the moon or from a green-legged tree partridge delivering its surprisingly tuneful message from the depths of a south-east Asian rain forest. In some species the function is solely to declare ownership of a territory and in others solely to attract a mate, but in many it is to achieve both.
The harsh churring of the nightjar, the instrumental bleating of the snipe and the wing-sound proclamation of the ruffed grouse, while none of them is tuneful, are all 'songs' in the biological sense.
The more typical songbirds belong to the sub-order called the Oscines, or true songbirds. They comprise almost half the birds that exist - over 4,000 species - and include, for example, such families as the thrushes (Turdidae), larks (Alaudidae), wrens (Troglodytidae), mockingbirds (Mimidae), the Old (Sylviidae) and New World (Parulidae) warblers and so on.
The songs of true songbirds vary from the simplest, such as the two-note “tune” of the great tit to the 103-note phrase (sung in as little as 8.25 seconds!) of the wren. If those species with the simple songs are saying the same thing as those with more complex songs, why the added complexity? There is no evidence that singers of complex songs are conveying additional information.
One bird from the many that sing long and varied songs is the sedge warbler. This bird employs sound elements that are the equivalent of piano notes (though less pure tonally) in the sense that they can be used in different temporal combinations. An individual cock sedge warbler may have as many as 100 of these syllables to play with - others have fewer. By recording a series of individually identifiable cock sedge warblers' outpourings, and by noting down how long it took each one to woo a mate, it became clear that the better singers secured mates sooner than the less accomplished songsmiths.
This expresses what is known as the evolutionary principle of sexual selection. The sedge warbler's song is a kind of acoustic peacock's tail. If you are a male sedge warbler and want to maximise your chances of getting a mate, then you just sing better.
Property and sex
In America, it has been shown that mockingbirds with wider repertoires not only attract females earlier, they also have bigger and better territories. Female mockers are attracted by elaborate songs, male mockers are more intimidated by them.
Property and sex are clearly important, and this importance is reflected in the tremendous amount of effort put into singing by the cock birds. A male yellowhammer may chant “a little bit of bread and no cheese” 3,000 times a day, half a million times in a season, and according to the physiologists, singing takes almost as much energy as flying. If you watch a yellowhammer delivering a song, you can see how he puts everything into it, his whole body quivering with the exertion.
Male and female duets
In certain bird species, the mated pair sing duets - male and female cooperate in proclaiming property rights. The role of each singer is so precise, and the two “parts” so closely coordinated, that it is hardly possible to believe that what you hear is from the throats of two individuals. The best-known and best-studied duettists are certain African shrikes. The male slate-coloured boubou shrike, for example, produces a couple of deep notes - “clack clack” immediately followed by a rising whistle from the female - “whee!". The two contributions are so beautifully synchronised it is hardly possible to believe that more than one bird is involved.
Recordings of such species as these, and the resulting voice prints, show that the partners' reaction time is to be measured in a tenth of a second. Just why such a complicated proclamation should have evolved is a mystery.