WILLOW WARBLER
ferred, although level ground is occasionally chosen, and in a flat country must often be resorted to. As a rule one egg is laid every twenty-four hours, but it sometimes happens that a day is missed, in which case the loss of time is made up by two being produced during the ensuing twenty-four hours. Six or seven eggs represent the normal clutch, but if the first nest is destroyed, the second clutch may consist of only three or four eggs. Incubation lasts from twelve to thirteen days.
It is possible to tell from the behaviour of the birds when the young are hatched, since there is a marked increase in the excitement they then show. This excitement may be said to be almost equal to that shown during the period of sexual activity. The period of incubation is a quiescent one; the male has nothing to do except to find food for himself, and for the female it is a time for rest. The energy to some extent exhausted during sexual activity is replenished, and upon the appearance of the young finds an outlet. A comparison of the manner in which this energy asserts itself at different periods forms one of the most interesting studies in bird life, a study to which in the future considerable time and attention must be devoted if we are to attain to a fuller knowledge of the meaning of the complex modes of behaviour, which are comprised under the term "expression of the emotions." I have just mentioned that, in the case of the Willow Warbler, the excitement shown at the period of parental care almost equals that at the period of sexual activity, and I do not think this is an exaggeration, for it would be a difficult task to draw a line between the two, or even to point to a single action which could be regarded as characteristic of only one period. It may be remembered that during sexual activity both sexes flap their wings in a peculiar manner. Upon the birth of the young I have seen the parents behave similarly; the female when near the male flapped her wings in the manner indicative of a desire for coition, and what is more curious still, coition appeared to be the result. But it is perfectly clear that this could not have
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