The disadvantages which the late arrivals have to face are therefore great, and it is probable that the percentage which attain to reproduction will on the average be somewhat lower than the percentage in the case of the earlier arrivals. The district in which my observations have been made lies well within the limits of the breeding range of most of our common species, and it is not surprising that I should have met with little evidence of failure to breed as a result of failure to secure territory. Some interesting information was supplied to me, however, by the late Robert Service. He found, in certain seasons in Dumfriesshire, flocks of from ten to fifty unmated Sedge-Warblers, which, from the time of their arrival in May until the middle of July, haunted reed-filled spaces along stagnant streams. These flocks appeared to him to be composed of loosely-attached individuals of a migrant flock that had failed to find things congenial enough to entice them to disperse. But may they not have been composed of males that had failed to secure territories, or of females that had failed to discover males in possession of territories, or of both?
We have seen that, in the case of many species, each male establishes itself in a particular place at the commencement of the breeding season, even though this may mean a partial or perhaps a complete severance from former companions. We must now discuss this fact in greater detail because it is opposed to the views often held regarding the sexual behaviour of birds, and is manifestly of importance when considering the theory of breeding territory.
First, however, there is a point which requires some explanation. I speak of the _same_ male being in the _same_ place. How can I prove its identity? In the first place it is highly improbable that a bird which roams about within the same small area of ground, makes regular use of a certain tree and a certain branch of that tree, and observes a similar routine day after day, can be other than the same individual. But, apart from this general consideration, are there any means by which individuals of the same species can be identified? Well, there is variation in the plumage. Supposing we take a dozen cock Chaffinches and examine them carefully, we shall find slight differences in pattern and in colour--more grey here or a duller red there, as the case may be--and though these differences may not be sufficient to enable us to pick out a bird at a distance, they are nevertheless conspicuous when it is close at hand. Then again there is variation in the song; and the more highly developed the vocal powers the greater scope there is for variation. But even the phrases of a simple song can be split up and recombined in different ways. If one were asked casually whether the different phrases of the Reed-Bunting's song always followed one another in the same sequence, the answer would probably be that they certainly did so, whereas the bird is capable of combining the few notes it possesses in a surprising number of different ways. And lastly, there are differences in just the particular way in which specific behaviour, founded upon a congenital basis, is adapted by each individual to its own special environment. Racial preparation determines behaviour as a whole, but the individual is allowed some latitude in the execution of details which are in themselves of small moment--the selection of a particular tree as a headquarters and a particular branch upon that tree, the direction of the distant excursion, and the direction of the limited wanderings within the small area surrounding the headquarters which in the course of time determine the extent of the territory, are matters for each individual to decide when the occasion for doing so arises. Moreover instances of abnormal coloration or abnormal song are not rare, and they are valuable since they place the identity of the individual beyond dispute. I can recall the case of a Willow-Warbler whose song was unlike that of its own or any other species, and of a Redbreast whose voice puzzled me not a little. I can recollect also a male Yellow Bunting whose foot was injured or deformed. Of this bird's behaviour I kept a record for two months or so; and inasmuch as it inhabited a roadside hedge, and was of fearless disposition, the deformed foot could plainly be seen whenever it settled upon the road to search for food. Identification is not, therefore, a difficulty. There is always some small difference in colour or in song, or some well-defined routine which makes recognition possible.