Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/25

 of which they defend against intruders ; they construct nests so similar that if it were not for the fact that those of the former are placed upon the ground and those of the latter slightly above it they would be indistinguishable, but even this slight difference does not appear to be absolute since the Willow Warbler sometimes builds above the ground. They make use of similar materials, frequently tearing off the bark of the honeysuckle which they weave into the foundation of their nests ; in the manner in which they feed their offspring and clean the nest there is little or no difference, and their behaviour after the young are able to fend for themselves is identical. So it is with the Beed and Marsh Warblers. They possess territories which they defend so long as the young require their care ; their nests are similar in construction and remarkable in type, the only difference being in the material used — which is due to the fact that they do not live in a similar environment — and in the manner in which they rear their young there is no perceptible difference. Much the same may be said of the Blackcap and Garden Warbler. Both possess territories, defend them, and remain in them until the young are fully fledged ; both build in similar situations and construct nests which differ, if at all, but little, either as regards type or material; and both adopt similar methods in the care of their offspring. In the case of the two White- throats the nests are placed in slightly different situations, but beyond this the instincts connected with reproduction do not differ. So here we have four separate examples in each of which the reproductive instincts so nearly approach one another that there is nothing to prevent our regarding the species from this aspect as continuous forms. But besides these special instincts there are those connected with migra- tion and bodily sustenance, and they are uniformly alike not only in those cases of close relationship, but practically throughout the whole genus. The laws of migration which apply to one apply to all, the same type of food is consumed, and the methods adopted in securing it are alike. We have