Black-collared Swallow Atticora melanoleuca
Six or more birds above the canopy with large group of swifts after heavy rain -- long forked tail, blackish above, white belly, black under tail, black across upper breast -- long tail and black-and-white pattern were very conspicuous. Black-collared Swallow is out of known range (closest Mitu, Colombia) and habitat (rivers with rocks). Blue-and-white Swallow has shorter tail and no black on forward underparts (southern migrants possible in July, birds from slopes of Andes also possible particularly since White-collared Swifts and Chestnut-collared Swifts were present at the same time). Barn Swallow has white in tail and lacks black/white pattern below (northern migrants not expected in July). Lesser Swallow-tailed Swift has similar long tail but completely different pattern below. Fork-tailed Palm-Swifts (present at the same time) also have long forked tails but completely different flight and pattern above and below. The birds in question were slightly bulkier than the palm-swifts, otherwise about the same size. Black-collared Swallow seems very unlikely -- but perhaps we do not know everything about this species. More likely is Blue-and-white Swallow (possible if I misjudged tail length and black pattern on breast). I had previously seen all of these species but Black- collared Swallow not recently.