256 The Naturalist in La Plata, 



ants, to catch the insects flying up from the earth 

 to escape from their enemies ; swallows also learn 

 to keep company with the traveller on horseback, 

 and, crossing and recrossing just before the hoofs, 

 they catch the small twilight moths driven up from 

 the grass. 



To return to the subject of voice. The tree-creepers 

 do not possess melodious, or at any rate mellow 

 notes, although in so numerous a family there is 

 great variety of tone, ranging from a small reedy 

 voice like the faint stridulation of a grasshopper, 

 to the resounding, laughter-like, screaming concerts 

 of Homorus, which may be heard distinctly two 

 miles away. As a rule, the notes are loud ringing 

 calls ; and in many species the cry, rapidly reiterated, 

 resembles a peal of laughter. With scarcely an 

 exception, they possess no set song ; but in most 

 species that live always in pairs there are loud, 

 vehement, gratulatory notes uttered by the two 

 birds in concert when they meet after a brief 

 separation. This habit they possess in common 

 with birds of other families, as, for instance, the 

 tyrants ; but, in some creepers, out of this confused 

 outburst of joyous sound has been developed a 

 musical performance very curious, and perhaps 

 unique among birds. On meeting, the male and 

 female, standing close together and facing each 

 other, utter their clear ringing concert, one emitting 

 loud single measured notes, while the notes of its 

 fellow are rapid, rhythmical triplets ; their voices 

 have a joyous character, and seem to accord, thus 

 producing a kind of harmony. This manner ot 

 singing is perhaps most perfect in the oven-bird, 



