Update Oct 08: added pics of female/juv birds at the bottom:
I presume this to be an aberrant male Purple Martin at Bentsen Rio Grande State Park/WBC, Hidalgo county, Texas on the late date of October 07, 2009 (it was with three female/juvenile-types) - any explanation for the white vent/abdomen patch?:
This is one of the female/juvenile types; the belly area appears to be unmarked white:
Here is a different individual (it has some damage to the right wing) showing the fork of the closed tail in flight:
