BROOD-CARE IN BIRDS 157 



for the whole period of incubation, although cooling for a short 

 time, such as happens when the brooding bird leaves the nest to 

 feed, does no harm, and is even imitated by those who are most 

 successful in using artificial incubators. The time required varies 

 from about ten days (in some of the small singing birds) to about 

 six weeks or two months (ostrich). On the whole it is rather 

 shorter in small birds than in large birds, at least within the same 

 families. Thus the small ducks require about three weeks, geese 

 about a month and swans about forty days. Comparing groups 

 with groups, there is much difference which is not easy to explain, 

 but the most general arrangement is that when the eggs are small, 

 containing little food-yolk, and the young are hatched in an im- 

 perfect condition, the period of incubation is short, as, for instance, 

 amongst passerine birds, and where the eggs are relatively large, 

 containing much food -yolk, and the young are hatched in a more 

 advanced state of development, the period occupied is longer. 



All the megapodes or brush turkeys have given up brooding the 

 eggs. Some of them lay in the warm sand, others are said to select 

 the neighbourhood of hot springs, whilst others again, like the 

 brush turkey most familiar in captivity, make huge mounds of 

 vegetable matter, the fermentation of which supplies the necessary 

 heat. The cuckoos in the Old World, and the cow-birds in the New 

 World, foist their eggs on other birds for incubation as well as for 

 subsequent feeding, and not a few of the owls are suspected of the 

 same habit. 



In some cases the male bird performs the whole duty of incubation. 

 This happens in the cassowary, rhea, tinamous, phalaropes and 

 painted snipe, and it is amongst these that the curious cases occur 

 in which the males are more dully coloured than the females. In 

 many birds both sexes share the duty. The cock ostrich watches 

 over the hole in which the eggs have been buried, by night, whilst 

 the hen takes up the duty by day. The screamers work in shifts 

 of two or three hours each. When they bred in the London 

 Zoological Gardens, it was noticed that the cock bird acted as time- 

 keeper, and at the end of a watch used to come and push the female 

 off the nest. The emperor and king penguins lay their single eggs 

 on the bare ground, often in extremely inclement weather when 

 there are heavy storms of snow and severe frost. Each egg is 

 brooded on the flat feet of the bird, and a warm flap of skin and 

 feathers, specially enlarged during the breeding season, hangs 

 down from the abdomen and covers it like a blanket. From time 



