294 THE BIOLOGY OF BIRDS 



succession, the male bird will thus be continuously engaged 

 in the task of feeding his various broods of offspring " 

 (Whitman, 1919, p. 63). 



§ 2. Nest-Making 



Birds are not the only animals that build nests. The 

 male stickleback glues together the leaves or fronds of water- 

 weed and the lampreys make a nest of stones. The squirrel 

 makes a big nest in the trees, and the harvest-mouse fastens 

 its cradle to the stems of corn. Among insects also there 

 are many constructions which might be called nests. The 

 wasp's hanging house of paper serves also for the deposition 

 and hatching of the eggs, and for securing the safety of the 

 young during the period of nurture. But the word " nest " 

 always suggests the word " bird," and that is obviously 

 because brooding is characteristic of birds and a nest is an 

 auxiliary to successful brooding. 



The uses of nests are mainly (i) to conceal and shelter the 

 eggs and the young and the brooding bird ; and (2) to make 

 brooding easier and more effective. Sometimes the nest 

 has a prior significance as a pairing platform ; sometimes its 

 depth is advantageous in keeping the nestlings from prema- 

 ture excursions. Sometimes, when the collection of non- 

 conducting material is large, the dominant function is that 

 of keeping up a suitable temperature around the eggs and 

 the imperfectly warm-blooded young ones. A good example 

 may be found in the feather nest of the eider-duck. In other 

 cases, when the outside of the nest is carefully decorated with 

 lichens and even gossamer, the function of concealment is 

 emphasised. The thorny sticks in the large domed nest of 

 the magpie suggest defence. In short, the character of the 

 nest varies as this or that function is emphasised. 



Specificity. — Within narrow limits of variability, usually 

 traceable to lack of materials, the type of nest is specific. A 

 particular kind of bird builds a particular kind of nest and, 

 as a rule, no other. Blackbird and thrush are near relatives, 

 but their nests are quite different. Examples of plasticity 

 are known, but the main fact is specificity, and this points 



