Here's a series of adult-basic Herring Gulls
larus a. smithsonianus photographed by Ted Lee Eubanks
in Texas:
1) Galveston, March 18, 1990; the bill of this bird would not
look out of place of a 2nd-basic, but otherwise it appears to
be an adult:
2) Galveston, 18 March, 1990; This bird has a very bright bill-base:
3) Galveston 18 March 1990; this bird seems to have molted most
of its contour feathers into alternate; note the indication of
some black in the tail:
4) Galveston, 3
December 1989; this bird has a somewhat Thayer-like bill, and
is P10 fully-grown?: