But the caution needed in dealing with these negative instances is well shown by the example of the Crossbill. It is common to quote this bird as an example of abnormal habit in the physiological sense, since it usually pairs and sings in December and January, breeds in February, and hatches its young in March. Naumann has already pointed out the meaning of this: the bird performs the work of propagation and rearing precisely in the months in which its chief food, the cones of pines, are at their ripest and best, so that the parent birds find it then easiest not only to feed themselves, but to supply their young with the seeds which they convey to them in their crops."
It will be seen from this passage that Dr. Häcker, like Mr. Aplin, clearly separates the true autumn song, heard after the moult, from the winter song which often begins in November; and with this conclusion most field ornithologists will probably agree. As to the meaning of the winter song, he is not so clear; apparently he takes it as in part "voice-play," the result of abundant food and bodily comfort, and as having no immediate connection with breeding, but adds a useful caution suggested by the case of the Crossbill. My observations of last winter, so far as they go, seem to support both his explanation and his warning.
It was on November 17th, a very uncomfortably chilly day, that I first made a note of the great number of Thrushes in song. No doubt Central and Southern England had been visited by a large immigration from the north and east. It was dull, moist weather, chilly rather than cold, and unusually still. I am convinced, though I cannot prove it, that not only old males, but young birds, and even females, were using their voices to swell the chorus: every bird seemed to be making some sort of noise, and there was every variety of performance, from the full, clear utterance of the practised singer to the harsh and wheezy notes of the novice or the female. As I have already said, this vocal activity continued in full swing, without apparent diminution of the numbers, until December 8th, when I left Oxford for the Christmas vacation, the weather all the time being mild and damp. I did not observe any distinct sign of courting or sexual activity.
After leaving Oxford, I was at Kingham, in the north-west of the county, until January 18th, and continued my notes there.
