Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/61

 to speak, one accustomed to construct their nests on old and the other on new reeds. And, if this be so, it will to some extent explain the peculiarly erratic arrival of different individuals even in the same reed bed, the earlier comers being those which had adapted themselves entirely to the use of dead reeds. The actual number of stems to which the nest is woven varies very considerably; one bird will make use of three reeds, while another of no less than seven. But the number chosen must have been in the past, and must be still, so regulated as to ensure the safety of the young; and in a case of this description we need have little hesitation in attributing the result to selection. A nest woven to two reeds, if it were possible to construct it thus, would not be durable, and even when woven to three does not always appear to be sufficiently secure, yet this number is sometimes resorted to; and if three reeds be really sufficient, why, in so many cases, should four, five, six, or even seven be used? It may be suggested that where a small number of stems is used it is the first attempt of a young bird, and that as it finds the number insufficient to hold the nest securely, it profits by experience, and at the next attempt adds another and yet another stem. But to profit by experience, persistence, with varying effort, is necessary. "Repetition of trials with variation of procedure is a sort of perceptual experiment. The results of previous experiment determine and facilitate future action, inasmuch as unsuccessful modes of procedure are gradually eliminated and successful modes alone survive." A bird builds a nest but once a year, and therefore the conditions necessary are not really present in a case of this description, and to expect it to realise the cause of the failure of its first attempt, and to overcome its difficulty in advance, would be to raise the level of its whole behaviour from the perceptual to the ideational plane, which I do not for one moment believe we should be justified in doing. And whereas