Two gulls. Both in juvenile plumage. Are they the same? If not, how to tell them apart?
It’s clear on inspection that they are different gulls. The legs are similar, but the bills and overall structures are different. The bills on both birds are black, but the bird on the right has a larger bill that is angled at the bottom. The bird on the right is also larger (though this might be difficult to determine from the picture).
Let’s focus on the bird on the left. Short bill—possibly a Mew Gull? The juvenile Mew Gull has pinkish legs and a black bill. However, it has a dirty, grayish brown head, back, and chest. The back feathers have light tips, giving a scaly appearance. The wingtips on the Mew Gull are blackish, whereas this bird has little black in the wingtips. How about a California Gull? However, the legs are wrong. The juvenile California has brown to blackish legs. A Ring-billed Gull is another possibility, but during their first two years, they have a pink bill. The best fit for the bird on the left is a juvenile Thayer’s Gull, which is typically variable from overall light grayish brown to deep smoky brown. The back is marked with light brown and the wingtips are darker than the back. The bill is small (compared to a Herring or Western Gull) and black.
he bird on the right is more readily identifiable. Its wingtips are colored unlike any other gull's, being neither black nor white. Instead, they are a medium gray, not much different from the back color. This identifies the bird as a Glaucous-winged Gull. Note that Western Gulls hybridize with Glaucous-winged, but in such cases the wingtips tend to be more blackish.