xvii. io NEST-BUILDING 5°5 



and the elaborate and prolonged courtship and frequent copulation of 

 man, continuing without cyclical breeding-seasons for as long as the 

 pair remain together and rear their young. In birds, as in man, the 

 'procreation of children' is not fully accomplished by a single act 

 of fertilization. 



10. Nest-building 



As in every other aspect of their life we find the nest of birds varied 

 in many ways to suit different manners of life. It is suggested that the 

 habit of making a nest may have arisen from the 'sex-fidgeting' that 

 is commonly seen before, during, and after copulation. This fidgeting 

 may take various forms, including making a 'scrape' in the soil or 

 picking up pieces of grass, &c, after copulation. There is certainly, 

 in many birds, a close connexion between nest-building and copula- 

 tion. Ritual offering of nest-material is an important element in many 

 courtship displays and in some species (e.g. magpie) male birds may 

 build extra nests. 



The nest is therefore often at first a sex site and its position may be 

 chosen by the male. The actual building of the nest is done very 

 variously. Sometimes the male brings the material and the female 

 uses it. She may do the fetching as well, perhaps accompanied by the 

 lazy male; or he may have nothing to do with the whole business. The 

 nest is built by means of a limited number of stereotyped movements, 

 which are characteristic of the species. The integration of these move- 

 ments into a functional sequence of behaviour depends, however, on 

 experience. Nest building and copulation occur at about the same 

 stage of the reproductive cycle and both can be induced by oestrogen 

 (in canaries). 



The complicated forms of nest are found only in passerine birds; 

 in others it is usually simply a hollow in the ground or a heap of sticks. 

 The more elaborate nests show many protective devices ; in temperate 

 regions, where predators come largely from below, the nests are often 

 open, but where there are many snakes they are mostly domed or 

 hung from branches or provided with a long tubular entrance (weavers). 



In building the nest the bird follows a set pattern, laid down in 

 some way by the method of working of its brain and showing no sign 

 of foreknowledge of the result. Young birds, however, build rougher 

 nests than mature ones. Weaver birds reared by hand for four genera- 

 tions made perfect nests of a type which, of course, they had never 

 seen. On the other hand, it has been claimed that canaries deprived of 



