GENERAL SUMMARY AND CONCLUDING REMARKS
the younger as a possible explanation of the art of nest building. Tradition in connection with this particular aspect of behaviour does not appeal to me as a satisfactory solution, for we cannot, as I pointed out when discussing the nest building instinct of the Reed Warbler, assume that a young male and a young female never mate together; and even granting that such an assumption were in some cases correct, there is a still greater difficulty to be overcome in the fact that the females of certain species—the Willow Warbler, Wood Warbler and Reed Warbler for instance—are alone responsible for the work of construction. If then this wonderful art is neither begotten of experience nor is the outcome of tradition, how is it to be explained? I cannot explain how it is done; we seem forced to fall back on inherited instinct. I believe that the young bird is as perfectly equipped with the mechanism necessary to produce these beautiful pieces of architecture as many insects which are provided with the mechanism necessary for the performance of a complicated series of activities once and once only in their lives.
Regarding as a whole the species whose lives have been dealt with in this work, we find that there is no one rule which summarizes the behaviour of the sexes as to the care of their offspring. The males of one species share the burden equally with their mates, the males of another take little part in the rearing of their family. It is the males of the three smaller species, the Willow Warbler, Wood Warbler and Chiff-chaff, who for some reason are not called upon to work so diligently as their mates, and these three build dome-shaped nests. Can we establish any relation between the behaviour of the male and the specific type of structure that has been evolved? It is difficult to do so with the life histories of a few species only to draw upon. But if there is one fact that the behaviour of the female, and in some cases of the male, does really seem to show, it is the necessity for constant brooding when the young have just left the egg. We can understand the immense importance of warmth; we need
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