Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/53

 that, in the case of many species, they are so seldom in evidence. But this we can understand if we remember that such birds would be restless wanderers, not resident in any one particular locality, but continually being urged to move from place to place by their sexual instinct; and, if at any time they should happen to be in or near a territory already occupied, they would be, no doubt, desirous of concealing their presence. This fact of the unpaired birds being so seldom in evidence, while at the same time so numerous, is one of some importance. It emphasises the close connection that must exist between territory and reproduction, for if there is no law of territory how can their behaviour be explained? Why should not the unpaired males instead of wandering about, apparently wishing to remain unobserved, be content to stay in one particular locality and make their presence known by song, as the majority of the males do who secure mates?

Many species have extended their breeding range in recent years and are even now extending it. It may be said that this extension is not real, but only apparent, owing to the greater interest taken in Nature by a consequently greater number of competent observers, so that whereas formerly a species, although present, remained unobserved, at the present time it is immediately detected. There is no doubt some truth in this, but at the same time there is strong evidence to show that in many cases the breeding range has unquestionably been extended. If the extension were limited to those species only whose numerical strength was exceptionally great, it would not, perhaps, be quite so surprising. But this is not the case; even those that are not numerous are slowly but steadily increasing their range. There is no direct necessity, so far as reproduction is concerned, for a bird to desert the particular neighbourhood in which it was reared; in fact, there would seem to be an advantage in its not doing so, since there would be a greater likelihood of securing a mate in a district already inhabited by the species; neither is there any