1872.] SWALLOWS OF BUENOS AYRES. 607 



breeding-season is over is to the broad leafy tops of an old ombee 

 tree ; and it is usually on these trees that they congregate, in parties 

 of from twenty to a hundred, before leaving us in February. 



If the species comprised in each genus or subgenus always resembled 

 each other as closely as the P. chalybea and P. purpurea, it would 

 be an easy thing indeed to classify ; for I am not acquainted with any 

 two distinct species more nearly resembling each other than these 

 birds. The difference in the hue of the under-plumage and a di- 

 vergence in one of the breeding-habits separates them ; otherwise 

 they are identical. Several times I had seen the P. purpurea in 

 Buenos Ayres, usually a single individual seen after midsummer, 

 associating with parties of the P. chalybea, and in size, language, 

 and flight so exactly like it, that I, not knowing the bird, was almost 

 inclined to think it a rare variety. 



On arriving at Bahia Blanca last summer I found the P. purpurea 

 quite numerous there, and the only large Swallow in that region, 

 the range of the other species of Progne not extending so far south. 

 Again, at Carmen de Patagones I observed great numbers of them. 

 They arrive there late in September, and leave before the middle of 

 February,' breed under the eaves of houses or in walls, and build a 

 nest like that of the P. elegans. But numbers also breed in the holes 

 in the steep clay- and sand-banks of the Rio Negro. Judging from 

 the appearance of all the breeding-places I examined, I am of opinion 

 that they never excavate holes for themselves, but merely take pos- 

 session of old forsaken burrows of quadrupeds and of the Burrowing 

 Parrot (Conurus patachonicus). I have remarked that the two species 

 described are identical in language ; the loud shrill excited scream 

 when the nest is approached, the various other short notes when the 

 bird sweeps about the air, and the pleasingly modulated and leisurely 

 uttered song are all possessed by the two species without the slightest 

 difference in strength or intonation. This circumstance appears very 

 remarkable to me, because, though two distinct species do sometimes 

 possess one or more notes alike, the greater part of the language will 

 always be found different, and also because I have noticed that in- 

 dividuals of one species in different localities do vary more in language 

 than in any other particular. 



In widely separated districts on the Pampas I have observed a con- 

 siderable difference in the notes of birds of the same species, particu- 

 larly in the songs of song-birds. I paid great attention to this 

 matter while in Patagonia, and in several species common to that 

 region and to Buenos Ayres found so great a difference in voice, that 

 I was fully convinced that birds have a greater tendency to vary in 

 language than in any thing else. It is, however, worthy of remark 

 that it is in resident species only that I have noticed this tendency 

 to vary ; the language of a passage-bird seems everywhere the same. 

 I may at another time have more to say on this subject. 



The third species, Progne tapera, is more slender, and has a greater 

 extent of wing than the two birds described; and instead of the 

 beautiful steel-blue (their prevailing colour), his entire upper plu- 

 mage is dull dusky brown, the under white. But if these differences 



