THREE UNPUBLISHED PAPERS BY BLYTH. 303 



appear to roost and build gregariouslj^ in dark cavernous recesses 

 of rocks, buildings, or the hollows of trees— some few in the 

 fronds of palms ; the nest is either formed of light substances 

 collected during flight, and more or less cemented together by a 

 viscid secretion of the oesophageal glands, or it is even wholly 

 composed of tliis substance, constituting then the " edible birds' 

 nests " so highly prized by the Chinese and Japanese. They are 

 extremely tenacious of their breeding-places, and, if undisturbed, 

 resort to the same nests season after season; if the nest be 

 removed, another is immediately constructed in the same place, 

 and this several times successively. Occasionally a number of 

 nests in contiguity are agglutinated to one another; and often 

 they are shaped like a cup vertically divided, the perpendicular 

 surface to which they are attached rendering a deposit of material 

 unnecessary on that side. The eggs are from two to four in 

 number, of a lengthened shape, and (as in the Trochiliclce pure 

 white in every known instance.* In general the Cypseliclce are 

 highly gregarious, often flying in companies, which are particu- 

 larly active and noisy when the air is highly charged with 

 electricity ; and they generally seek their insect-prey high in fine 

 weather, and near the ground when the weather is damp and 

 gloomy, and towards the dusk of evening — conveying pellets of it 

 to their young, accumulated and held beneath the tongue as in a 

 pouch. Their voice is simple screaming or twittering, according 

 to the species ; and the young of Cypselus apus has been remarked 

 to emit a singular continuous sound, "like the humming of a tea- 

 kettle." The flight of all is most vigorous and powerful, smooth 

 and sailing in some species, more or less rapid and impetuous, or 

 in others flickering and irregular, yet with such power of wing it 

 is remarkable that the geographical range of the species is not 

 particularly extensive, but .in general rather the reverse. Ex- 

 amples of this family occur in most countries, except the coldest; 

 and at all elevations from the plains to the mountains; where the 

 winter is cold they migrate. In Britain it is remarkable that 

 the Common Swift {Cypselus apus) and Nightjar {Caprimulgus 

 europmis) are the latest of all the summer visitants to arrive, and 



* I suspect that the eggs described by Capt. Tickell in J. A. S. xvii. 30'2, 

 are erroneoiisly ascribed by him to the Palm Swift, even if found in the nest 

 of that bird. Capt. Tickell, however, remarks that the nest was not brought 

 to him. 



