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