160 BIRD BEHAVIOUR 



which also takes longer, need not be mentioned, 

 because the wild fowl is not so large as this bird, 

 and curiously enough increase of size in domestica- 

 tion, however great, does not seem to affect the 

 incubation period to any noticeable extent. 



There does not seem to be any general correlation 

 between the activity of the chick and the period of 

 incubation ; the young of the Hmipodes or Button- 

 Quails, birds about the size of Quails proper, hatch 

 in only twelve days, although downy and active, 

 while the helpless nestlings of the Budgerigar, an even 

 smaller bird, take nearly three weeks, Parrots being 

 a long-period group ; and the nestlings of birds of 

 prey require a long time to hatch, the largest, the 

 Condor, even requiring six weeks. On the whole 

 there is a tendency for water-fowl to take a longer 

 time than land-birds, no aquatic groups having a 

 really short period, though none take so long as 

 some of the great runners. The period of incuba- 

 tion does not vary more than a day or two unless 

 over a month, when the variation may be a matter 

 of several days, as in the case of the Swan and 

 Ostrich. 



As the temperatures of different groups vary, 

 incubation may be hastened artificially in some 

 cases ; when Mandarin Ducks were first bred at 

 the Zoo, now nearly seventy years ago, it was 

 noticed that those of their eggs which were put 

 under hens hatched two days earlier than those 

 which the Ducks themselves were allowed to sit 

 upon. M. Rogeron, in discussing Hens as foster- 



