Page:The British Warblers A History with Problems of Their Lives - 2 of 9.djvu/53

 This uniformity seems to extend to all the activities, whether referable to instinct or habit; but at the same time there are incidental details connected with them which vary in each particular case, and in what manner they are carried out can only be determined by each individual as the need arises. It is while performing these details that I have noticed a remarkable tendency towards the formation of fixed habits, a tendency to repetition. I mention this here because it may bear some relation to the law of uniformity. It is of common occurrence in Nature. I have referred to it in the way in which the Chiff-chaff approaches the nest, but other birds have the same habit. When searching for materials to build their nests, birds frequently return to the same spot, although such material could be gathered more conveniently; and in the same way they return again and again to the same place for food; the Great Spotted Woodpeckers (Dendrocopus major) are a good instance of this, for they often have their rounds, consisting of certain trees, which they visit in order daily; families of Tits also have their rounds. We find the same thing in the accurate measurements of periods of time, but more conspicuously in the way in which the same branch in a certain tree in the breeding territory is utilised as the headquarters. A better illustration is the manner in which the same birds repeatedly return to the same situation for the purpose of breeding. A number of Martins (Chelidon urbica) build against my house in such a position that their nests are washed away by any heavy rain, yet they return year after year. It is impossible to prove that the same birds return each year, but this is really unnecessary, since the nests