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The language of birds: 1.1 The dawn chorus

Robin (Erithacus rubecula) (27K)

Robin Erithacus rubecula. Robins sing both to repel rival males and to attract a female.

As is well-known, the songs of birds are delivered most frequently each day for a period of perhaps 40 minutes after daybreak. In my own garden in the west of England, on let us say the 1st May, the song thrush Turdus philomelos may strike up well before dawn, soon he will be followed by the robin, blackbird T. merula, wren Troglodytes troglodytes, garden warbler Sylvia borin, chiffchaff, hedge sparrow Prunella modularis and chaffinch. There is no fixed order in which each species takes its cue from the eastern sky, but there is a genuine tendency for some to start earlier than others. Indeed wren, robin, song thrush and blackbird often burst into song well before daybreak, this natural phenomenon has given rise to such phrases as "up with the lark" and "rise before cockcrow". Incidentally, the "cock-a-doodle-doo" of the rooster is both a territorial proclamation and an invitation to a hen! Also this early bird song is known as the dawn chorus - or even as the birds' "hymn to the dawn". In fact the birdsong is as much a battle hymn as a love song! It seems that individuals seeking territories are particularly active immediately after first light, for it is light enough to get about but not light enough to forage. It is, in any case, a cold time of day and the prey of the insect feeder is not yet active. So confrontations take place but once there is enough light to feed by, both challenger and challenged are presumably diverted by the need to satisfy their hunger. The dusk-chorus is a lesser event but there is a very noticeable increase in song each evening in the British spring, in my garden at least the song thrush seems to be particularly defensive at this time of day.

It is also true that songs carry much better around dawn and dusk because there is less wind at those times of day, not only is there tranquility, there is also much less background noise (natural and man-made). For these two reasons a typical bird song, it has been calculated, will carry 20 times as well as at noon.

Listen
Robin Erithacus rubecula
song, recorded by Richard Ranft, 14 February 1993, Fernhurst, West Sussex, England.

There are about 4,000 true songbirds in 53 different families, and, as may readily be imagined, their songs 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 in my garden. 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 composes a seemingly endless stream of constantly varying phrases, so let us examine a voice print of part of one of its songs and imagine that it is a sheet of music. Time is from left to right, and frequency (pitch) is from bottom to top on each line. It will be seen that the 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.

Sedge warbler sonogram (23K)

A sonogram of the complex song of the sedge warbler Acrocephalus schoenobaenus.

Vertical axis = frequency scale in kiloHertz.
Horizontal axis = time in seconds


Listen
Sedge Warbler Acrocephalus schoenobaenus
song, recorded by William Pedley, 25 April 1981, Wicken Fen, Cambridgeshire, England.

By tape 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! 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.

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