1 86 A HISTORY OF BIRDS 



of South America. It is a remarkable structure, thick-walled, 

 globular and of great size. For the most part made of clay 

 and dung, the builders, however, evidently understand the value 

 of hair and long grass as binding materials, for these are worked 

 into the mass just as hair is used in mortar in the construction 

 of human dwellings. In section it is found to contain a central 

 chamber, with a long passage running partly round it, the inner 

 chamber being lined with fine grass. Since the materials for 

 building can only be obtained in damp weather this edifice 

 takes several months to build. It is further to be noted that 

 while three distinct species all build precisely similar nests, two 

 adopt a more conventional style of nursery. One of these, 

 Fnrnarius figulus, builds a nest of sticks, while the other, F. 

 torridus, breeds in holes in banks. 



Among the builders of the more elaborate nests where finely 

 divided materials are used, cement, in the form of a secretion of 

 the salivary glands, is commonly employed. This is the case, 

 as we have already seen, in the nests of Salvin's Swift (p. 184). 

 But there are certain Swifts, of the Genus Collocalia — a genus 

 extending from the Indo-Malayan countries and Australia — 

 which build their nests almost, or entirely, of inspissated saliva. 

 These birds attach their small saucer-shaped cradles to the walls 

 of caves, generally mixing grass or feathers with this secreted 

 matter. But the species known as the Esculent Swiftlet {Col- 

 localia fucipJiagd) commonly dispenses with all foreign materials 

 and uses the products of the salivary glands entirely, and it is 

 these nests which furnish the material for the " birds'-nest soup" 

 considered so great a delicacy by the Chinese. The collection 

 of these nests constitutes an important industry, as may be seen 

 from the fact that from Borneo alone, over three and a half 

 million nests have been exported in a single year. As might be 

 supposed, these birds build in huge colonies, and while in some 

 cases the caves are approached by water, in others the floor is 

 dry and covered with a deposit of guano varying from eighteen 

 to thirty feet thick ! 



No less remarkable are the nests of some of the Tree-swifts 

 of the Genus Macropteryx since these are made up of a salivary 

 secretion intermixed with bits of bark, while in point of size they 

 represent the smallest nests known — relatively to the size of the 

 builders. About an inch and three-quarters across and half an 



