Like many large gull species, American Herring Gulls exhibit very little to no sexual dimorphism in their plumage. This means that adult males and females are nearly identical in their feather coloration and patterns, making it extremely difficult, if not impossible, to reliably determine their sex in the field based solely on appearance.

General Appearance of Adult American Herring Gulls (both sexes):
Size and Build: They are large, robust gulls, typically larger than California Gulls but similar in size to Western Gulls, though often appearing somewhat less stocky than Westerns. They have a relatively large head and a heavy bill with a pronounced gonydeal angle (the angle on the lower mandible).
Plumage (Breeding/Summer in Oregon):
Head and Underparts: Clean white.
Mantle (Back and Upperwings): A medium-gray mantle, darker than Glaucous-winged Gulls but paler than Western Gulls.
Wingtips: Black wingtips with prominent white "mirrors" (spots) on the outermost primaries. The pattern of black and white mirrors can be useful for distinguishing them from other large gulls, but not for sexing.
Tail: White.
Plumage (Non-breeding/Winter):
In non-breeding plumage (seen in fall and winter, and by some younger birds in summer), the head and nape develop variable dusky streaking, creating a "smudged" or "dirty" appearance. The streaking can extend to the breast. Both sexes develop this.
Bill: Yellow, with a prominent red subterminal spot (a red spot near the tip of the lower mandible). The bill is heavy.
Legs and Feet: Pinkish. (Note: European Herring Gulls, *Larus argentatus*, often have yellow legs, a key difference).
Eyes: Typically pale (yellowish) with a reddish eye-ring.

Subtle Differences (Males vs. Females):
1. Size:
Males tend to be noticeably larger and heavier than females on average. This difference can be most apparent when observing a mated pair side-by-side, where the male might appear more robust, with a larger head and a proportionally heavier bill.
However, there's considerable overlap in size between individual males and females, so trying to sex a single bird based on size alone is often unreliable. A very large individual might be male, and a smaller one female, but it's not definitive.

2. Bill and Head Proportions:
As part of the overall size difference, males may have a slightly larger, broader head and a more substantial bill compared to females. The bill's depth and total head length are often slightly larger in males.

3. Behavior (especially during breeding season):
Territoriality and Aggression: Males are typically more active in defending their breeding territories.
Courtship: Males perform various courtship displays, including head-tossing, "long calls," and "choking" displays. Males also engage in **courtship feeding**, regurgitating food for the female. Observing this behavior is a strong indicator of sex.
Copulation: During copulation, the male mounts the female, providing a clear visual cue to their sexes.
Incubation and Chick Rearing: Both parents share incubation duties (typically 2-3 eggs) and care for the chicks.

Important Note on Identification in Oregon:
It's crucial to correctly identify American Herring Gulls in Oregon, as they can be easily confused with other large gulls, especially **Western Gulls** (which have darker mantles and often dark eyes) and Glaucous-winged Gulls (which have pale, concolorous wingtips). Identifying the species correctly is the first step before even contemplating sex. Also, hybridization, particularly with Western Gulls or Glaucous-winged Gulls, can occur, leading to individuals with mixed characteristics that defy easy classification, let alone sexing.