THE CEMENT OF BIRD MASONS 173 



materials are cemented together, we must first con- 

 sider the nests of the swallows and swifts. These birds 

 secrete a very sticky saliva, which quickly hardens 

 when it is exposed to the air. This constitutes an 

 excellent cement. Watch a swift working at its nest 

 under the eaves of a house. It flies to it with a feather 

 or piece of straw carried far back in the angle of its 

 mouth, hangs itself by means of its four forwardly 

 directed toes on to the half-completed nest, which is 

 stuck on to the wall of the house, and, having carefully 

 placed the feather or straw in the required position, 

 holds it there until the sticky saliva it has poured over 

 it has had time to harden and thus firmly glue the 

 added piece of material to the nest. The bits of straw, 

 feathers, etc., may be said to constitute the bricks, 

 and the saliva the cement of the swift's nest. Some 

 swifts build their nests exclusively of their saliva. 

 These constitute the " edible birds' nests " of commerce, 

 and may be likened to houses built entirely of cement. 

 The martin (Chelidon urhica), the common swallow 

 (Hirundo rustica), and the wire-tailed swallow (i^. 

 smithii) construct their nests of clay and saliva. They 

 repair to some puddle and there gather moist cla}^, 

 which they stick on to some building, so as to form 

 a projecting saucer-shaped shelf. In this the eggs 

 are laid. But nature has not vouchsafed sticky saliva 

 to all birds, so that many of them have to find their 

 cement just as they have to seek out the other building 

 materials they use. 



The chestnut-bellied nuthatch [Sitta castaneiventris) , 

 which nestles in holes in trees, fills up all but a small 



