Slaty-backed Gull is a coastal species that breeds from the Kamtchatka Peninsula to northern Japan. In the Pribilofs, these gulls are rare in spring and uncommon in fall, Slaty-backed Gulls make investigating the sitting hordes of Glaucous-winged Gulls worthwhile. In May and June this species is rarely encountered, but always possible. Many of the spring sightings are of first or second year birds, which can be tricky to separate from young Herring Gulls. By mid July, third year and adult birds dominate, and usually one or two individuals can be found with concerted effort every week. Slaty-backed Gulls of an age that is advanced enough to show adult-colored mantle feathers can be separated from the Vega Herring Gull (the default Herring Gull in the Pribilofs) by the darker mantle (blackish gray as opposed to dark gray) and usually yellow iris. The separation of first year Slaty-backed and Vega Herring Gulls can be problematic. Look for brownish primaries and tertails with pale fringes, wholly dark tail, plain greater wing coverts and often a very faded look to the general plumage. Vega Herring Gulls should show barred greater wing coverts, blackish primaries and tertails, and pale-based tail with a terminal band. Look for Slaty-backed Gulls in amongst flocks of Glaucous-winged Gulls, especially in the Salt Lagoon, Big Lake and Northeast Point.