248 THE STORY OF THE BIRDS 



are worth their weight in silver, and are found 

 in caves in limestone and volcanic rocks ; those 

 from caves in a sandstone formation being of 

 inferior value. There are usually three crops 

 of these nests gathered in a season, the second 

 being the best. Their value depends upon their 

 purity or freedom from foreign substances, such 

 as dirt and feathers. The best quality nests are 

 white, the second quality is yellow, whilst the 

 third is discoloured and impure. The nests of 

 some other of the Swifts are very interesting. 

 A Guatemalan species is said to attach its tubular 

 nest to a rock with saliva, the nest itself being 

 made of seeds, each seed cemented together with 

 a similar secretion. 



The last family in this order famous for the 

 nest-building capacities of its members is the 

 Trochilidae. It is somewhat remarkable that 

 the Humming-Birds should not only excel as 

 nest-builders, but should depart so widely from 

 the general rule prevailing in the order as to 

 make elaborate structures in the open. As we 

 have already seen, the majority of the Coracii- 

 forms breed in holes, and make but slight nests, 

 or even dispense with nests altogether, laying 

 their eggs upon the powdered wood only. Some 

 of the most exquisite nests that are known are 



