5 o8 BIRD BEHAVIOUR xvn. 12 



response in a part of the skin, the brood spots, produced by a local 

 moult of the feathers. At hatching the parents may assist the action of 

 the caruncle of the young. The care of the nestlings is a very elaborate 

 business in most birds, involving many separate actions. The young 

 are warmed, fed, and occasionally watered, and in many species the 

 nest is kept clean by careful removal of the faeces, which may be 

 produced as pellets enclosed in a skin of mucus; these, being shining 

 white, are eaten by the parents. In warm climates the parents may 

 shield the nestlings from the sun during the heat of the day. 



The work of caring for the young birds is often performed by both 

 parents and there are various adaptations to ensure this. The young 

 react strongly to the return of the parents to the nest, usually by open- 

 ing the beak and displaying the coloured inside of the mouth, an 

 action that strongly stimulates the parent, releasing the feeding 

 behaviour, which varies with the species. The young pigeon thrusts 

 its bill into the throat of the adult to collect the milk secreted by the 

 crop. It is probable that this careful attention by the parents is 

 ensured by a series of somewhat simple stimulus reactions. For 

 example, Eliot Howard showed that a female linnet responds to its 

 own nest rather than to its own young. When its young were put into 

 an abandoned nest and the latter placed near to its own the hen 

 usually returned to its nest and neglected the young, though the male 

 gave them some food. Cuckoos similarly make use of this undiscrimin- 

 ating 'instinctive' behaviour; birds will feed any young that provide 

 the appropriate stimulus and if no young appear they will make little 

 effort to find them. 



The rate of development after hatching varies greatly. In the gal- 

 linaceous birds, in many ways a primitive group, the young are well- 

 developed at hatching and soon fend for themselves (nidifugal). In 

 nidicolous species, on the other hand, the young is naked and helpless, 

 it is a growing machine, with a large liver and digestive system but 

 little developed nervous system. Yet the birds of these species ulti- 

 mately have much larger brains and are more 'intelligent' than those 

 that leave the nest soon after hatching. 



