Update, May 07 2007: compare the bird below to this Larus michahellis from the UK just seven days later, temporily. There are slight differences, but it is an extremely close match. Some/most of the small differences may be due to slight morphological differences between source populations of these two gulls (i.e. if the TX bird is a YLGU, it may be from an Iberian/Macaronesian population that averages slightly darker on the head).
This first-cycle gull was photographed March 29, 2007 in the Coastal Plain near Houston and Galveston, Texas, Texas:
