BRITISH WARBLERS
of arrival of the females is often the cause of much quarrelling; for although she seems to pair with the owner of the territory in which she first makes an appearance, yet throughout the first morning or the first day of her arrival she does not confine herself so strictly to the boundaries as she is wont to do later on. It is clearly impossible for her to know the limits of the territory in which she has settled; she must learn by experience; and this experience is probably gained under compulsion, for a male can sometimes be seen pursuing a female, who has crossed the boundary, and driving her back again into his territory. Her arrival in a territory is the cause of extreme sexual excitement on the part of the male who is its owner, and, if she crosses the boundary, on the part of the adjoining male. This excitement expresses itself similarly in both sexes, and results in much pursuing and frequent mock conflicts. During the pursuit the flight is often rapid, the birds twist and dart in and out of the bushes, momentarily settling, but only to commence their round again. These outbursts of excitement are by no means continuous, but spasmodic, and may be commenced by one sex or the other. Thus the female at one moment appears to call the male by singing, when he either approaches her slowly, or flies at her more abruptly, uttering at the same time a quiet purring sound. This act of flying at her is of common occurrence, and when it takes place she may on his approach spread her tail, assuming an attitude of defence which results in a fluttering of tiny wings and a momentary clicking of bills. Or he may fly towards her apparently with the object of presenting her with food—an attention on the part of the male which is common amongst many species—and he then warbles quietly or rather sings his song continuously. When the male overtakes her during the pursuit, both birds may fall fluttering to the ground in their excitement.
The manner in which sexual emotion is expressed varies in different, and even in closely allied, species. Why it should be so we cannot tell; yet the difference is often striking. I
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