BLACKCAP 



could a bird recognise, even by the light of previous experi- 

 ence, 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. Dur- 

 ing 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 



* The notorious case quoted by Eomanes in his " Mental Evolution in 

 Animals," of a bird which having placed its nest upon a forcing house, only 

 returned to sit upon the eggs at night when the temperature fell, is corro- 

 borative evidence, but I think wrongly ascribed by that great writer to 

 intelligent modification. 



21 



