THE SWALLOW TRIBE. 241 



at the base, and with the upper mandible hooked, and some- 

 times the lower recurved a little, so that the sides do not shut 

 so tightly as to crush any soft substance that may be between 

 them ; and that, with the wideness of their gape and gullet, 

 enables them to carry a cargo to their nests when they have 

 young. These characters apply chiefly to those that feed by 

 day : the twilight ones, of which there is only one British 

 species, have their plumage much softer, and vibrissa, or 

 hairs, dependent over the sides of the bill, with muscles at- 

 tached to them, capable of communicating impressions as well 

 as receiving them : these may help the owners to feel their 

 way, and also to entangle the insects, as the birds cannot, 

 from the extent to which the bill opens, and the position of 

 the eyes, have much use of them in finding their way. 



In some of the systems, these birds are classed as fissi- 

 rostres, that is, " open-bills," as the bill does not shut at the 

 sides ; but that is only one part of their structure, and one 

 that is not an immediate or general key to their habits.* 

 The following figures will give some idea of the bills of the 

 nocturnal and the diurnal Chelidonian birds, both of which 

 differ from that of the bee-eater and other birds that pur- 

 sue the larger insects upon the wing. 



Goat-sucker. Swallow. 



These birds fly for their food in the air, something in the 

 same manner as the whales swim for theirs in the water; 



* The term fissirostres does not mean " open-bills," but applies to the 

 extent of the gape, enabling the bird to open the mandibles widely. M. 

 VOL. I. R 



