1 82 BIRD BEHAVIOUR 



entrance walled up till only sufficiently large to let 

 the owner in ; and just as the Martin's nest is 

 exaggerated into a bottle by some foreign Swallows, 

 so the Syrian Rock-Nuthatch carries out the 

 entrance-plaster into a regular cone, using resin as 

 well as mud in the construction. 



Generally speaking only passerine birds are skilful 

 nest-builders, nest-building, even more than song, 

 being the " strong suit " of this great group ; but 

 among some non-Passerines we find fairly remark- 

 able nests, especially the strange secreted nests of 

 the edible- nest Swiftlets (Collocatia). The isinglass- 

 like secretion of which the species whose nests are 

 used in commerce is made (CoUocalia francica) is 

 simply the solidified saliva ; the flow of this secretion 

 is very copious in nesting Swifts, and many gum 

 their nests together with it, but even the other 

 Collocalias usp extraneous materials, and so render 

 the nest commercially useless. At the time of 

 writing, the edible nests could even be bought in 

 London. 



The handsome Eastern Crested Svvifts {Dend.ro- 

 chelidon), which differ from other Swifts in perching 

 on boughs like Swallows instead of clinging to 

 vertical surfaces, build a very curious Uttle saliva- 

 nest at the side of a bough, just large enough to 

 hold their single egg, on which they do not actually 

 sit in the nest, but perch on the bough and cover 

 their treasure with the breast. Such a nest is 

 naturally exceedingly hard to find. 



Another Swift, from Central America this time 



