164 THE BIOLOGY OF THE SEASONS 



mixed with dung, or in the nest of the mistle-thrush, which 

 has a considerable foundation of mud, we have instances 

 of the numerous connecting types between hard-built and 

 felt-work nests . The tailor-birds make a thread of vegetable 

 fibre, and sew together the edges of a couple of leaves ; the 

 fantail warbler also uses a thread knotted at the end to 

 bind grass-stems into a canopy over its nest. Of certain 

 warblers (Aedon and Thamnobia) it is said that they in- 

 variably lay a piece of snake's slough in their nests, like the 

 horseshoe by the house-door. 



The series of typical nests might be greatly prolonged, 

 but we must bring it to a close with a reference to what is 

 perhaps the most extraordinary of all nests that of the 

 sea-swift (Collocalia). The bird occurs in great numbers 

 in Indian and Australian regions, and usually nests socially 

 in caves, both by the sea and among the mountains. The 

 peculiarity of the nest the well-known " edible bird's- 

 nest " is that it normally consists of the dried secretion 

 (largely mucin) of the salivary glands. When the first 

 nest has been gathered, the bird sometimes builds an 

 inferior type of nest, including a considerable quantity of 

 vegetable matter glued together with the hardened salivary 

 juice. It is worth recalling the fact that not a few of our 

 British birds moisten with their saliva the vegetable fibres 

 and the delicate twigs which they use for nest-making, 

 thereby rendering them more pliable. It seems likely that 

 the inferior nests of Collocalia, in which vegetable fibre 

 predominates, represent for this genus a primitive rather 

 than a degenerate type. 



THE NATURE OF THE NEST is SPECIFIC 



We have seen that there is a long inclined plane from no 

 nest at all to the most elaborate nest, but the important fact 



