Thayer's Gull is a species that has long caused identification challenges for birders, particularly in Oregon where it winters. Adding to the complexity is the fact that it was formerly considered a subspecies of Iceland Gull and is still sometimes grouped with it by some authorities.

Regardless of its taxonomic status, when it comes to distinguishing between male and female Thayer's Gulls, it's important to understand that they exhibit minimal to no sexual dimorphism in their plumage. This means that adult males and females look virtually identical in their feather coloration and patterns, making them extremely difficult to sex reliably in the field based on visual appearance alone.

General Appearance of Adult Thayer's Gulls (both sexes) - focus on winter plumage as that's when they are most common in Oregon (July is very early for them to be in Oregon, usually fall/winter migrants):
Size and Build: They are medium-sized gulls, generally considered to be intermediate in size between a California Gull and an American Herring Gull. They have a relatively rounded head and a proportionately smaller, more "gentle" or "finer" bill than a Herring Gull.
Plumage:
Mantle (Back and Upperwings): Medium-gray, similar to but often slightly darker than an Iceland Gull, and sometimes slightly paler than an American Herring Gull.
Wingtips: This is a key identification feature. The wingtips have a limited amount of black, usually appearing as dusky smudges or blackish markings that are paler on the underside of the primaries when seen in flight (a critical difference from Herring Gulls, which have extensive, opaque black on the undersides of their primaries). There are usually small white "mirrors" on the outermost primaries.
Head and Underparts: In non-breeding (winter) plumage, the head and nape typically show variable amounts of dusky, often "smudgy" streaking. The rest of the underparts are white. In breeding plumage (less commonly seen in Oregon), the head would be clean white.
Tail: White with faint to distinct dusky barring, especially in younger birds.
Bill: Yellow with a variable reddish subterminal spot on the lower mandible, often with an indistinct blackish ring. The bill shape is often described as finer or more slender than a Herring Gull's.
Legs and Feet: Pinkish.
Eyes: Typically dark (blackish-brown). This is often a good field mark to distinguish them from American Herring Gulls (which usually have pale eyes). However, some Thayer's can have paler eyes, and some Herring Gulls can have darker eyes, so it's not foolproof. They usually have a pink orbital ring.

Subtle Differences (Males vs. Females):
1. Size and Structure:
Males tend to be slightly larger and heavier than females on average. This might manifest as a male having a slightly more robust body, a larger, broader head, and a proportionally thicker or longer bill.
Some experienced observers note that female Thayer's Gulls often conform more to the classic "dove-headed" appearance with a very rounded head and a relatively dainty bill, while males can be quite robust and look surprisingly similar to American Herring Gulls in overall structure, albeit with the critical primary pattern differences.
However, the overlap in measurements and individual variation is significant, making it extremely difficult to reliably sex a single bird in the field based on these subtle structural differences alone.

Challenges in Identification in Oregon:
Taxonomic Confusion: The taxonomic history of Thayer's Gull is complex, and some authorities now lump it with Iceland Gull (as the *thayeri* subspecies of Iceland Gull). This can be confusing when consulting different field guides or resources.
Hybridization: Like other large gulls, Thayer's Gulls can hybridize, particularly with Western Gulls and Glaucous-winged Gulls. These hybrids can further complicate identification and obscure any subtle sexual differences.
Juvenile Plumages: Gulls take several years to reach adult plumage, and their immature plumages are often highly variable and challenging to identify, let alone sex.