62 EVOLUTION OF BIRD-SONG 



chirp noisily every morning except when frost is 

 severe ; and then, if the cold is intense, not a single 

 chirp can I hear ; but directly a thaw comes, all the 

 birds at once announce the change of temperature. 

 The nightingale, grasshopper-warbler, sedge-warbler, 

 and landrail may be mentioned as singing at unusual 

 hours. 



The stimulus which induces birds to sing particu- 

 larly at dawn may be the same as that which leads 

 them to the same exercise in the rain the pleasure 

 of anticipating approaching incidents : in the one 

 case, daybreak, which brings comparative safety to 

 the waking creatures ; and in the other, the coming 

 forth of many kinds of insects and larvae. The 

 insect-eaters, at least, would be able to appreciate 

 the latter event, and that may possibly account 

 for their singing more than the finches during rain. 

 There is nothing to show that erotic emotions are 

 in any way excited by dawn or by rain. The habit 

 of singing early and late may also have been en- 

 couraged by the enforced leisure which many birds 

 must then undergo, when they cannot seek food, 

 and when their young need no sustenance. The 

 nightingale's habit of nocturnal singing is easily 

 understood : the male sings to attract the migrating 

 female, and does not seek her. She flies by night, 



