32 BIRDS OF LA PLATA 
with tremulous flutterings, short and rapid as those 
of a butterfly. 
Neither is this bird gregarious like all its congeners, 
though occasionally an individual associates for a 
while with Swallows of another species; but this 
only when they are resting on fences or trees, for as 
soon as they take flight it leaves them. Once or twice 
when for some mysterious cause the autumnal migra- 
tion has been delayed long past its usual time, I 
have seen them unite in small flocks; but this is 
very rare. As a rule they have no meetings pre- 
paratory to migration, but skim about the fields and _ 
open plains in un-Swallow-like solitude, and in a 
little while are seen no more. 
RED-BACKED ROCK-MARTIN 
Petrochelidon pyrrhonota 
Above glossy dark steel-blue; lower back and rump cinnamon 
rufous; upper tail-coverts brown; wing black; tail black, glossed 
with green; crown steel-blue; forehead sandy buff; cheeks and 
sides of face chestnut; chin chestnut and lower throat steel-blue ; 
fore-neck, chest, and flanks ashy brown ; middle of breast and abdomen 
white, tinged with brown; length 5.3 inches. Sexes alike, 
THIs species does not breed in the Plata district, and 
is only seen there in spring, flying south or south- 
west, and again in much larger numbers on its return 
journey in autumn. Nor does it breed anywhere in 
South America, so far as we know, but in Arizona 
and other districts in the northern division of the 
