212 ANIMAL LIFE 



make their eyries on cliffs, crags, or trees, constructing 

 them chiefly of branches and sticks. Pigeons make 

 but a slight foundation of twigs. Rooks adopt a rough 

 lining of roots, and jackdaws improve on this by lining 

 their large nest, hidden in some hole, with fur. 



With small birds, whether solitary or colonial, the 

 case is different. Their choice of site is wide, and 

 the degree of secrecy and skill employed in the making 

 is most varied. The nest is usually of the open-topped 

 type, and generally consists of an outer layer of 

 coarse material lined with moss, feathers, or some 

 fine and non-conducting material. Both sexes assist 

 in its construction, and employ their bills and feet in 

 doing so with a skill and assiduity that reach their 

 maximum in the exquisitely finished handiwork of the 

 titmouse (fig. 43) and the chaffinch. 



The need for this greater skill in constructing and 

 concealing the nests of small birds seems to lie not 

 only in the defencelessness of the artificers, but in 

 a consideration that affects their young. Nests are 

 formed, not merely to surround and screen the nestlings, 

 but to enable the incubating heat of the sitting bird to 

 be concentrated on the eggs. Now the size of an animal 

 is directly related to its power of retaining heat ; for 

 with two creatures, one double the weight of the other 

 but of equal proportionate powers of heat production, 

 the heat produced will be as eight to one, whereas the 

 loss of heat in proportion to weight, that is, roughly, in 

 proportion to production, will be determined by the 

 amount of body surface from which heat is lost, that 



