Bird species and their songs
With song, members of each species communicate only with other members of their own species. Indeed, one of the more elementary functions of song is to enable females to recognise males as being of the “right” species.
Willow warbler song
The song of a willow warbler recorded in Hampshire, England, by Michael Strand.
Wood warbler song
The song of a wood warbler, recorded in Herefordshire, England, by Victor Lewis.
Song of a chiffchaff, recorded in Dorset, England, by William Pedley.
Individual male songs
Song is not only specific to each species. In some species at least (and perhaps in many) each individual male singer can be identified by other males by his song. Make a recording, convert it via a sonagraph into a “voice print” and one has, in effect, a finger print, a sound signature. Not only can we tell individual cocks apart in this way - so can the cocks themselves by listening to one another.
This was first proved in the case of a New World warbler called the ovenbird. Imagine bird A settled in its territory, with three neighbours 1, 2 and 3. All four are singing normally and none worries unduly about his neighbours. If, however, you introduce the taped voice of another bird, B, (previously recorded from a territory several miles away) playing it back within earshot of A, then A instantly recognises the “bird” as a stranger and at once investigates. Individual signature songs and the ability of ovenbirds to recognise them, functions to save territory holders time and energy. Once identified there is no need to investigate birds who are “next door”, because experience has shown how they usually operate.
Ovenbird song, recorded in Algonquin Provincial Park, Ontario, Canada by Tom Cosburn.
This is not to suggest that neighbours are necessarily well behaved. Bird 1 could well set out to annexe part of A's territory, but A would presumably notice him visually or detect that his song sounded clearer. Thus it is important to know that your neighbour is keeping - or not keeping - his distance. The loudness of his song will give a crude measure of his distance.
To help territory holders of certain species to monitor the behaviour of neighbours, a special technique has evolved. Certain species like the chaffinch and the great tit do not sing exactly the same phrase each time they sing. Each individual has a repertoire of several phrases, many of which it shares with its neighbours. If you are in the presence of either of these species it soon becomes noticeable that each individual which initiates a bout of singing is almost invariably answered with the phrase most closely resembling the one that he himself used. The counter singer, in other words, matches the song he hears.
Why? Well, the suggestion is that by this means the answerer (and for that matter the initiator) can best judge the distance that separates the two. Each knows what that precise phrase sounds like to himself as he sings it, and can therefore, if the same phrase is used back, make the best possible comparison, determining the degree of degradation in the signal and thus the distance of the signaller.
It may sound a little far-fetched, but it is certain that song-matching takes place, and this is one plausible explanation.