Glaucous-winged Gull x Western Gull	Thayer's Gull

DISCUSSION
On the hybrid, note the muddy ill-defined upperpart patterns, the large bill, and the overall bulky size of the bird. On the Thayer's, note the more defined checkering and the small straight bill. Most hybrids have more solid greater coverts than this one. Many Thayer's have more patterned tips to the tertials (evident on the upper tertials here); this is rarely seen on hybrids. These birds are easy to distinguish; many others are more intermediate. For example, some hybrids have pronounced checkering and smallish bills.

Besides the size difference (which is not always apparent), the best ways to tell a first year Gl-WxWestern hybrid from a Thayer's Gull are not visible in these pictures:
1) the tail: Gl-W/Western/hybrids have solid tails, maybe with faint marbling at the base of the outer retrices from below in good light; Thayer's have pale barring at the base of the outer retrices and pale barring up the outermost web of the outer retrices (folded under here).
2) the underside of the primaries: Gl-W are solid pale silvery; Thayer's are similar but show a thin dark trailing edge to the outer primaries (what I call the "Arctic Tern effect"). Take a look at these illustrations of this.

photos courtesy of Don DesJardin, copyright 1998