MARSH WARBLER
is manifest, since they are either essential to its welfare or necessary for the attainment of reproduction. Every bird must search for and find sufficient food; again, every bird must be in possession of a suitable territory, and most birds must build a nest if incubation is to be carried out successfully, and any individual that came into the world imperfectly endowed with just those qualities requisite for a proper carrying out of the routine of activities, would either succumb or fail in the attempt at reproduction. The life of a bird is not, however, wholly spent in searching for food or in making preparations for its progeny; each species, in addition, behaves at certain definite times in certain definite ways, and it is this behaviour which we will now examine, limiting the investigation to those species whose peculiar but characteristic attitudes it is the purpose of these plates to demonstrate. It would be difficult to find a family more suitable for such an investigation, for, on the one hand, the nervous system of its different members is so framed as to produce a visible emotional behaviour seldom surpassed in bird life, and, on the other, the secondary sexual characters are not highly developed. The combination of these two characteristics in the same individual is important; for since the emotional behaviour reaches its highest degree of intensity, and the secondary sexual characters their greatest development during the period of reproduction, a direct relation between the two has always been deemed more than probable.
The period of reproduction is the period when behaviour is most emotional, and part of that period we often find referred to as one of courtship. We read of "the courtship being a prolonged affair," or that "courtship may thus be regarded, from the physiological point of view, as a means of producing the requisite amount of pairing hunger," or, again, "that it is the instinctive coyness of the female that necessitates all the arts of courtship," but of the term courtship no explanation is offered. The word is indefinite, implying a period of
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