MARSH WARBLER
is incorporated with the song, but often a phrase or a number of phrases from a highly specialised song—sometimes even a complete song of simple type—and where phrases are introduced those phrases are in their proper sequence. Let anyone attempt to combine and recombine without pausing an equivalent number of tunes, and he will find that some effort is required to do so. There is much yet to be learnt about the power of imitation. For how long is a bird able to retain an alien song in its memory, and to what extent does it add to its store as the season advances? Nothing in the way of a decisive answer can be given to these questions. But the key to the first is to be found in the answer to the second. When the males first reach their destination do they reproduce as many alien cries as they are accustomed to a few weeks later in the season? That they do so is scarcely likely; for as the days pass by new notes must be so frequently copied as to become associated with the song, thereby extending the list of imitations. The tendency to reproduce alien phrases must be founded on a congenital basis. Given this tendency, the rest, it may be said, will follow in due course. There is much to be said for this contention. We observe the acquirement actually taking place in the case of birds kept in captivity, and we find that such acquirements are so retained as to be subsequently reproduced; we also observe a similar process at work in Nature, as when the cry of some species is immediately reproduced by the imitator. At one time or another every imitation must have been an act of individual acquirement. Now we start with a congenital basis, of the origin of which we are completely ignorant. The young bird born in June leaves this country a few weeks later without having had an opportunity of exercising its vocal powers, and probably unacquainted with its ancestral song. In the winter months it hears the songs of many foreign species, of some of our own summer migrants, and possibly even of its own species. With the rise of the sexual instinct its vocal powers begin to develop, and
59
