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 im common 
with birds of other fainilies, 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 of 
singing is perhaps most perfect in the oven-bird, 
