California Gulls are a common sight in Oregon, often found along the coast, at inland lakes and reservoirs, and even in agricultural fields. They are medium-sized gulls, readily identified by their dark eye, yellow-green legs, and a bill that has both a black ring and a red spot near the tip.

When it comes to distinguishing between male and female California Gulls, it's important to note that they exhibit very little sexual dimorphism in their plumage. This means that, based purely on feather coloration and patterns, adult males and females look virtually identical.

General Appearance of Adult California Gulls (both sexes):
Size and Build: Medium-sized gull, generally larger than a Ring-billed Gull but smaller than a Herring or Western Gull. They have a relatively slender build with a somewhat rounded head.
Plumage:
Head and Underparts: Clean white. In non-breeding plumage (late fall through winter), they may develop some dusky streaking on the head and nape.
Back and Wings: Medium-gray mantle (back and upperwings).
Wingtips: Black wingtips with prominent white "mirrors" (spots) on the outer primaries.
Tail: White.
Bill: Yellow with a distinct black ring near the tip and a small red spot on the lower mandible, often at the gonys (the angle on the lower mandible). The red spot can be brighter in breeding adults.
Legs and Feet: Yellowish-green.
Eyes: Dark (blackish-brown). They typically have a thin reddish eye-ring.

Subtle Differences (Males vs. Females):
1. Size:
Males are typically slightly larger and heavier than females. This difference can be most apparent when observing a mated pair side-by-side, where the male might appear noticeably more robust, with a larger head and a slightly thicker bill.
However, there's overlap in individual sizes, so a single large gull isn't necessarily a male, nor is a small one necessarily a female. Morphometric studies (using precise measurements of bill length, bill depth, head-bill length, and tarsus length) have shown significant statistical differences, but these are difficult to apply reliably in the field.

2. Bill and Head Features:
Following from the size difference, males may have a proportionally larger bill and a broader head with a flatter crown compared to females. Again, these are subtle differences best observed when comparing individuals, especially a known pair.

3. Behavior (especially during breeding season):
Courtship Feeding: During courtship, the male often offers food to the female (courtship feeding). This can be a strong indicator of sex.
Territoriality and Aggression: Males tend to be more overtly aggressive in defending their breeding territory.
Incubation and Parental Care: Both sexes participate in incubating the 2-3 eggs and raising the young. In many gulls, males take a larger share of night incubation, while females do more during the day, but this can vary.
Copulation: During copulation, the male mounts the female. While often brief, this is a clear indicator of sex.