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282 impetus of the descent. It may be a matter of impetus, but to me it has often seemed more as though the sound gave the idea of impetus, or added to it, and that the sweeps were, sometimes, just as impetuous, or even more so, when made without it. As I observed, the birds flew to their trees at a very moderate height—not very much, indeed, above the trees themselves—and, whilst many made the whizzing sound, the great majority swooped down without it. It seems, therefore, to be a special sound produced by the rooks at pleasure, and always accompanying an excited frame of mind. First one bird and then another gets excited, and dashes suddenly down with the whirring or whizzing noise, so that, as the sound is not vocal, and is only heard upon such occasions, it has all the appearance of being caused by the quick, sudden motions of the wings. But it is possible that some particular way of holding the quill-feathers of the wing or even tail is required to produce it, in combination with the general movements, and this would account for its being sometimes heard and sometimes not heard, when these latter are identical. The curious burring note is likewise, but far less frequently, an accompaniment of these wild excited sweepings, and this is most often the case when they are from a considerable height. Here, again, the note bears a clear relation to the bird's mental state, so that it would appear that the degrees of pleasurable excitement cannot be estimated by the motions alone.