The 24 September 2004 World Birding Center pelagic trip out of Port Isabel was fortunate to run across a Red-billed Tropicbird calmly sitting on the water.  This bird was originally identified as an immature White-tailed Tropicbird until a few hours later on the return to dock when Tony & Phyllis Frank started examining their photos and began questioning the bird's identification.

There are 6 photos below - the first two are courtesy of  Tony Frank, the remainder are mine.  Note in the flight shots the black on the outer wing extends thru the primaries into the primary coverts - which appears to be diagnostic for Red-billed Tropicbird.