Update November 29, 2009: added comparisons to Chimney Swifts photographed at the same time and place - at the bottom of this page:
This Chaetura swift was flying in loose association with a handful of Chimney Swifts
at Mitchell Lake Audubon Center, Bexar County, Texas on August 09, 2009; for comparison there are two photos of Vaux's Swift I photographed in Guatemala (most likely the migratory subspecies rather than the smaller, darker sedentary form from central Mexico) at the bottom of the page. This individual stood out as being slightly smaller, much paler on the underparts and rump/utcs, and by its more-fluttering flight - unfortunately, none of the swifts vocalized during my encounter with them:
For further comparison here are some of the pics of the Mitchell Lake swift mixed with pics of Chimney Swifts C. pelagica (labelled "pelagica") taken at the same time/place:
Note the "Roman Nose" bill of the pelagica below, compared to the swift above:
in the comparisons above and below note the different stage of primary molt between the target swift and these two different pelagicas:
Below I've taken the first (best) image of the San Antonio swift, reduced it, then overlayed it onto the photo of a Vaux's Swift from Guatemala (the Guatemala original followed by the overlay); note the EXACT match of wing and body proportions...:
