Bridled/Sooty Tern Onychoprion anaethetus/fuscatus was at Calaveras Lake, San Antonio, Bexar County, Texas on September 07, 2010, along with a handful of Black Terns.
I had originally thought that it was a Bridled, but when I looked at my photos soon after the event i convinced myself that the apparent dusky under-primaries meant it must be a Sooty.
Thanks to the rapidly expanding eBird archive of photos it is clear that in certain situations it is not rare for the underside of the primaries on Bridled Tern to appear dusky, and thus I am identifying this bird as a Bridled Tern - See the following examples:
A distant bird from Louisiana.
A close bird from Egypt.
another close bird from the Philippines.
The contrastingly pale, mid-gray rump and uppertail (evident in at least two of the photos, and seen in the field) is a feature only seen on Bridled, not Sooty. Similarly the somewhat isolated dusky on the head is much better for Bridled than Sooty.
Although Bridled is less-expected after storms than Sooty, the difference in local records is not great: three regional records of Sooty on eBird versus none for Bridled, but I am aware of one non-eBird record from San Antonio by a very experienced local birder.