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first cycle
Thayer's Gull hybrid


see proposed thayeri-kumlieni-glaucoides scale
Fig 1) Note the checkered upperparts, including the greater coverts, the patterning on the tertial tips, and the dark brown (not black) folded primaries with pale crescents formed by their pale tips. Dec 21, 1998. Southern California. Photo courtesy of Don DesJardin.
Fig 2) This one has exceptionally patterned tertials. Note the smooth brown head. Thayer's Gulls keep this nicely checkered juvenile plumage into February, later than most gulls. Dec 21, 1998. Southern California. Photo courtesy of Don DesJardin.

Fig 3) This one has tertials almost as dark as the primaries. Note the dove-like head; this may be a female. That's a first year Glaucous-winged in the rear left and Western Gulls immediately behind and on the right. Late November, 1998. Southern California. Photo courtesy of Don DesJardin.

Fig 4) Note the dark brown tail with some white marks at the base of the retrices. March 22, 1998. Santa Barbara, California. Photo courtesy of Don DesJardin.

Fig 5) Here's a really well-patterned bird. Note the touch of pale in the bill, which is uncommon but not extremely unusual. This bird is oiled above the left leg. Dec 2, 1998. Southern California. Photo courtesy of Don DesJardin.

Fig 6) Note the pale inner webs and darker outer webs of the flight feathers; also the extensive pale markings on the outer rectrices. The large pale terminal areas of the rectrices are a common characteristic, but is not shown by all birds. December 10, 2001. Ventura, California. Photo courtesy of Don DesJardin.
Fig 7) From the underside, note the pale wings but also the thin dark trailing edge to the outer primaries, similar to a tern. Glaucous-winged Gulls do not show this. October 10, 2000. Lake Winnipeg, Manitoba. Photo courtesy of Ann Cook.

Figs 8-10) This is a dark, but nicely marked, bird. While the brown parts are rather dark, the patterning is still perfect for Thayer's Gull. November 30, 2010. Lake Mendocino, California. Photo courtesy of Matt Brady.

Fig 11) Finally, in late March, Thayer's begin showing some moulted scapulars with some adult gray in them, as well as some head and body molt. Otherwise, this bird is still in juvenile plumage, which is very typical. Note the much blacker primaries of the Herring Gull behind it (back right). March 23, 2011. Clearlake, Calfornia. Photo courtesy of Steve Hampton.

Comparison with Glaucous-winged x Western Gull
Fig 12) Glaucous-winged Gull x Western Gull Fig 13) Thayer's Gull

Photos courtesy of Don DesJardin.

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 first cycle wing patterns.