Differences Between Ring-billed and Short-billed Gulls
September 29, 2024
Misidentifications I see on the Metro Vancouver eBird somewhat frequently involve Short-billed and Ring-billed Gulls, especially juveniles and first cycles (juveniles that have begun the first pre-alternate/pre-breeding moult).
Adults are the easiest to separate.
Ring-billed Gull gets its name from the black band towards the end of the bill. At most, Short-billed Gull will have dark markings in a similar position, but never a complete ring. Sometimes the bill tip of some Short-bills is yellow, and the basal portion greenish yellow.
Adult Ring-billed Gull in breeding plumage. Note black band around outer bill, pale yellow eye.
Adult Short-billed Gull in non-breeding plumage, undergoing prebasic/post-breeding moult. Head is all white during breeding season, like Ring-billed Gull. Note dusky, speckled eye.
Also, Ring-billed is larger than Short-billed, has a lighter grey mantle (upper body) and pale yellow eyes instead of dusky/speckled.
In flight, Ring-billed has smaller wing mirrors on p10-p9 than Short-billed Gull (white patches near the top of the outer two wing feathers).
Ring-billed gull in prebasic/post-breeding moult. Note limited wing mirrors on outer primaries.
Short-billed gull in prebasic/post-breeding moult. Note extensive wing mirrors on outer primaries.
The juvenile plumages of both species are reliably identifiable as well.
Ring-billed is whiter overall, with contrastingly dark brown patterning on upper body, wings and behind the tail. Short-billed is grey-brown overall, bleaching whitish over winter.
Both species's bills start off dark, then become pink at the base. The dark tip of Ring-billed is more clearly demarcated.