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 



