508 



NATURAL HISTORY OF BIRDS. 



gape split to near the eyes, but without bristles. Altogether they resemble closely 

 the swifts (Micropodidae), though structurally they are very different, and the distin- 

 guishing marks have already been pointed out (cf. page 437). Inter se, the different 

 species of swallows vary but very little, and no intermediate forms are known which 

 directly connect them with some other family, or indicate their line of descent. But 

 different as they appear externally from other Passeres, nothing is found internally or 

 even in their pterylography that will justify their being far removed from the forms 

 hitherto treated of. Says Professor W. K. Parker : " In this remarkable group of 

 tender-billed gaping Passeres, there is not, as far as I am aware, a single aberrant 



FIG. 249. Clivicola riparia, bank-swallow ; Progne subis, purple-martin. 



character of importance. The skull, the skeleton generally, the digestive and the 

 vocal organs all these might belong to species of the genus Sylvia," 



About one hundred species of swallows are recognized, distributed all over the 

 earth except the very arctic and antarctic regions. Uniform as is their structure, so 

 are also their habits, they being chiefly migratory, and feeding on flying insects which 

 they catch on the wing. Their whole organization aims at great speed, and they spend 

 the greater part of their life in the air, but rarely perching in order to rest. The 

 rapidity of their flight is only surpassed by its perseverance, while the shortness of 

 their feet prevent them from moving on the ground except in the most awkward man- 

 ner. In regard to their nesting habits, the swallows may be divided in three 

 groups : (1) Those which build their nests in hollows of any description, not dug out 

 or prepared by the birds themselves ; to this group belong different species of the 



