Page:The British Warblers A History with Problems of Their Lives - 3 of 9.djvu/47

 could a bird recognise, even by the light of previous experience, that there was any connection between a definite temperature of the eggs and the ultimate production of the young; or how could a young bird without experience know this? By some means this instinct is capable of being regulated by external influences, but by what means this actually takes place we do not know; we do know, however, that if it were not so, certain species that build nests exposed to climatic variations would in a series of cold springs suffer very heavily.

The eggs are usually five in number, one being laid every twenty-four hours. When incubating the male and female change places very frequently. The male leaves the nest in reply to a call from the female, and she then takes his place. While incubating the male often sings, in reply. I believe, to other males.

Incubation lasts about fifteen days, but it is probable that in the case of this and other species, the period may vary with the prevailing climatic conditions.

Few birds display more excitement during the period of incubation than the male Blackcaps; and when a number of pairs inhabit the same wood it is fascinating to watch the gatherings of the males, not only on account of the interest that always attaches to any attempt to investigate the ultimate cause of their actions, but also for the pleasure derived from a close study of the attitudes that result. During this fortnight or three weeks they are very noisy and sing continuously in the mornings, but not so much during the day. Soon after pairing has taken place a deterioration in their vocal powers commences and continues until their