An inland county with a salt lake is a gull magnet. The good news: adults sort out on size, mantle shade, wingtip, and leg colour. The honest news: young gulls take three or four years to grow up, and a flowchart can't finish that job — so this keys the adults and tells you where the ceiling is.
The first, biggest cut — before a single field mark.
Bulky, heavy bill, flat crown. The large Larus — take ~4 years to mature. California, Ring-billed, Herring, Lesser Black-backed, Western, Short-billed, and the pale white-wingers.
Dainty, small bill, rounder head. The hooded gulls — mature in ~2 years, buoyant flight. Franklin's, Bonaparte's, Sabine's.
Every mark below assumes an adult. A big gull cycles through mottled brown juvenile → 1st → 2nd → 3rd-winter plumages before it's clean-mantled, and those stages overlap between species. If your bird is brown and streaky, get the size and structure, then reach for a dedicated gull reference — the key can't finish it.
Tap through to a name. Assumes a clean-mantled adult.
The full key for adults — follow the arrows.
Mantle shade × wingtip × leg colour — the adult big-gull decoder.
Scroll the ramp palest → darkest. Two adult gulls of the same shade usually differ in leg colour or wingtip — that pairing is the fast clincher (California is the dark-mantled, greenish-legged one; American Herring the pale-mantled, pink-legged one). Relative mantle shade is a standard field reference (Sibley-style), not county data; the hooded gulls aren't on this scale.
Ordered common → rare. Amber box = the clinching adult mark.
Larus californicus
Utah's state bird and a Great Salt Lake breeder — the default big gull here. Greenish legs, darker mantle and dark eye split the adult from American Herring (see pairs).
Larus delawarensis
The common winter parking-lot gull. The complete bill-ring and yellow legs separate it from the bigger, pink-legged American Herring.
Leucophaeus pipixcan
A summer marsh breeder, abundant on migration. Bold white eye-arcs and a grey-centred underwing split it from Bonaparte's (see pairs).
Larus smithsonianus
The 2024 split of Herring Gull (now American Herring). Pink legs, pale mantle and pale eye separate the adult from California (see pairs).
Larus glaucoides
Now includes darker-winged "Thayer's" birds, so wingtip marking varies. Smaller and gentler-faced than Glaucous.
Larus fuscus
Increasing, but still document it. Yellow legs and a paler-than-black mantle separate it from the pink-legged, blackish Western (see pairs).
Xema sabini
A fall migrant, mostly juveniles, on open water. The three-block wing is diagnostic; on the water note the dark, yellow-tipped bill.
Chroicocephalus philadelphia
Buoyant, flitting flight over water. Thin black bill and the white wing-wedge separate it from the red-billed, grey-underwinged Franklin's (see pairs).
Larus hyperboreus
Bigger and heavier-billed than Iceland. Note: eBird records it here (~0.9%) but it isn't on the 2020 county checklist — report/verify rather than assume.
Larus glaucescens
Document it. Also a prolific hybridiser (see Don't import) — a grey-wingtip bird flecked with black may well be a cross, not a pure bird.
Larus brachyrhynchus
Formerly "Mew Gull" (2021 split). The dove-like head and stubby bill separate it from the sharper-faced, ring-billed Ring-billed (see pairs).
Larus occidentalis
Document it inland. Pink legs, a blackish mantle and a massive bill separate it from the yellow-legged, slate Lesser Black-backed (see pairs).
Adult pairs where careful birders still trip.
California vs American Herring
The core valley pair. California is darker-mantled with yellow-green legs and a dark eye; American Herring is bigger, paler-mantled, pink-legged, pale-eyed. Leg colour alone usually settles it.
Ring-billed vs California
Ring-billed is smaller and paler with a clean black bill-ring and a pale eye; California is bigger with a darker mantle, a dark eye, and a red-and-black bill mark rather than a full ring. Yellow legs on both.
Ring-billed vs Short-billed
Short-billed is smaller and gentler — a rounded, dove-like head and a big dark eye on a short, near-unmarked yellow bill. Ring-billed is squarer-headed, sharper-faced, pale-eyed, with a bold complete bill-ring.
Iceland vs Glaucous
Both pale white-wingers. Glaucous is big and heavy-billed (a bulldog); Iceland is smaller, rounder-headed, small-billed, with longer wing-projection. On a lone bird judge bill heft and structure, not just paleness.
Franklin's vs Bonaparte's
Franklin's has a bold black hood, thick white eye-arcs, and a dark red bill; Bonaparte's is smaller and more tern-like with a thin black bill, a winter ear-spot, and a flashing white leading-edge wedge. Underwing: Franklin's greyer, Bonaparte's whiter.
Lesser Black-backed vs Western
Mantle shade and legs. Lesser Black-backed is slate-grey (paler than black) with yellow legs and a slimmer build; Western is blackish, pink-legged, very bulky with a huge bill. Both are documentation-worthy here.
The age ceiling, the rarities, and the hybrids.
Everything in this key assumes an adult. A big gull spends three to four years in mottled brown juvenile and 1st–3rd-cycle plumages, and those stages overlap heavily between species — a flowchart genuinely can't finish them. Get size and structure, then reach for a dedicated reference (Howell & Dunn, Gulls of the Americas). Don't force a brown gull to a species just because the key wanted one.
On the county list, but the UBRC requests documentation. Photograph and report.
eBird shows these in the county; the 2020 UBRC checklist doesn't list them. Treat as unresolved, not automatic.
Gulls hybridise more than any group here — the county data alone holds Herring × Glaucous-winged, Western × Glaucous-winged, and several slashes.