These Biscutate Swifts Streptoprocne biscutata were part of a flock of 30+ birds feeding at the scenic overlook at Chapada dos Guimaraes; Bradley Davis had told us we might see them in this area, but none of my book references indicated that they occur this far west in south-central Brazil. I've seen an online version of the range map from HBW which shows them occurring much further to the west than do either the SWIFTS book or the recent BoSA-non-Passerines guide - but Chapada dos Guimaraes is a further 200+ miles northwest of the mapped HBW range.
These birds seem to have just started their primary molt. I was struck by the squarish tail and by how relatively obvious were the dark breaks in the white "collar" on each side of the neck; when the head was facing directly ahead (in line with the body) these dark breaks were angled rather than parallel with the bodyline; when the head was twisted towards or away from the viewer, the thickness of these breaks contracted or expanded. I was surprised by the very noticeable pale brownish - almost peachy-looking - "front" to the face, isolating the dark bill; Sick's description seems quite accurate in this regard, but while the SWIFTs books mentions that the facial area is paler, there is no indication of how prominent this is nor does the illustration show this (nor does the one in the BoSA-non-Passerines guide):