parts of the song of the Nightingale and the Garden-Warbler, amongst other species, being introduced. Now these males would not have left the nest until the middle of the previous June, and at this date both the Nightingales and Garden-Warblers are silent; one may, perhaps, hear occasionally a momentary outburst of song from a Nightingale, or a short and quiet warble from a Garden-Warbler, but even this soon ceases, and neither of these species then sing again before leaving this country. These males next return at the beginning of the following April, while the Garden-Warblers do not arrive until some weeks after this date, and the Nightingales about the same time, but they are generally rather later. It is, therefore, clear that they could not have acquired these imitations during the few weeks that they have been in contact with these other species in this country. But there remains the period between September and the following April, the time in which they are resident in their winter home. What happens during these months? My impression has always been that such species as the Nightingale and Garden-Warbler do not sing. If I am wrong in thus thinking, it is clear that they would have had ample opportunity of acquiring them during this period; on the other hand, if I am right, and these species, so far as their song is concerned, are silent, then we can only regard the imitations as congenital.
There are other points in connection with the imitative faculty which are difficult to explain except on the hypothesis of transmission. Why, for instance, in almost every series of imitations, should the song of the Nightingale and Garden-Warbler be introduced? It is true that the males have considerable opportunities of hearing them, but it is also true that there are other species inhabiting the same situations which are more frequent and more persistent singers. Or, how is it that the series of imitative strains is to so large an extent composed of the songs and call-notes of our indigenous species? As far as the song of our own migrants, in their winter home, is concerned, silence may reign, but the males
33
