120 SEX-LORE 



upon the male, the female being quite indifferent to 

 them, which is rather unusual among birds. The 

 young of this type of bird generally arrive in the 

 world in an almost mature condition, so that they 

 need on the whole little parental care. Yet they 

 suffer for want of it ; for in their early days they fall 

 a prey to many foes, not being sufficiently developed 

 to defend themselves. As a rule, where there is 

 a waste of material, there is a compensating in- 

 crease in production, and vice versa. Birds breed 

 on the average once or twice a year, and only lay a 

 limited number of eggs at each breeding season, 

 from one or two in some species, up to four or more 

 in others. It is interesting to note that the fertility 

 of the birds of the lower order is, on the whole, 

 larger than that of the higher order. 



A steady increase in the amount and quality of 

 parental care is to be found among the next class of 

 birds to be discussed, and with it also a corresponding 

 decrease in the number of offspring. To this class 

 belong the grouse and partridge, the quail, penguin, 

 petrel, and other swimming birds. Some of these still 

 lay their eggs in sand and gravel, just like the running 

 birds mentioned above; others, such as the penguins 

 and petrels, hatch their eggs between their thighs on 

 high rocky ledges. The emperor penguin, which 

 only lays one egg at a time, keeps it upon the upper 

 part of her foot, out of contact with the ice, and even 

 nurses the yov "ig there for a time (Fig. 43). A warm 

 flap of skin and 3athers, which becomes enlarged dur- 

 ing the breeding season, hangs from the bird's abdo- 

 men and protects the egg, and afterwards the young, 

 from the severe weather. The male and female take? 



