Thayer's Gull

The status of Thayer's Gull as a species is somewhat controversial. First described in 1915, it was long considered a subspecies of Herring Gull (Larus argentatus), but that is not the general view today. Now more commonly considered a distinct species part of a complex of gulls that includes as many as six species. Thought to be most closely related to Herring Gull (in this case, American Herring Gull, and to Iceland Gull (Larus glaucoides). The last of these three is usually not present in Sonoma County. Thayer's Gull is an uncommon wintering gull here, typically found in small numbers in mixed gull flocks between November and March (if noticed at all), although records exist from late October. Bolander and Parmeter call March 31 (1979) the latest local spring record, but their accompanying bar graph contradicts that, showing the bird's presence in small numbers through early May and the Ebird data shows sightings in late April (although this gull, is often misidentified). Thayer's Gulls, like Herring Gulls, attain adult plumage in their fourth winter phase--that is, when about three-and-a-half years old.

We mostly see juvenile birds or adult birds in non-breeding plumage--or at least these two stages are most often noted because they are easier to distinguish from similar species than birds intermediate in development. Winter, non-breeding adults are fairly large, typical gulls--larger than a California Gull (Larus californicus), somewhat smaller than the similar Herring Gull—with a smaller bill than Herring Gull, deeper pink legs (usually, but highly variable), and (usually) medium to dark brown eyes (Herring Gulls most often have very pale, yellowish, staring eyes, but the shades may overlap in the two species; eye color alone is not diagnostic). Thayer's Gull has a proportionately smaller, rounder head (the angle of the forehead is steep, whereas in Herring Gull it is longer and sloping--often said to give Herring Gull a "meaner" look in contrast to the more mild countenance typical of Thayer's. The head, neck, and upper breast of both birds show brownish-grey streaking in winter plumage but with subtle differences. The mantle is a medium-dark grey, usually slightly darker than a typical Herring Gull. Most sources say that Thayer's tend to have very dark slaty-grey primaries rather than pure black primaries, but they also note that the color is variable and often hard to judge. Many also point out that the white spots in the dark primaries tend to be linear rather than round, extending along the long axis of the feather, which gives the upper surface of the wingtip in flight a somewhat striped rather than spotted look with significantly paler undersides to the wingtips; Herring Gull wingtips will more closely mirror the upper surface coloration. The more of these features that exist in combination, the more secure an ID will be, but even "experts" will often argue about the ID of a particular gull, and separating Thayer's and American Herring Gull in Sonoma County is especially tricky. As always with gulls, approach any attribution with skepticism and review the books often.

In reality, Thayer's Gull can be very difficult to distinguish from similar species. In fact, while I'm personally fairly confident the photos on this page labeled Thayer's Gull are, in fact, Thayer's Gulls, I reserve the right to be wrong, and there are almost certainly birders that would argue that my example bird above is a Herring Gull. If your opinion differs from mine, please tell me why. It's all about learning.

Juvenile birds (photo below) have a few features that make them somewhat easier to identify. Look for: a proportionately smallish, uniformly dark chocolate brown bill on a fairly small, rounded head, usually with only minimal paleness at the base of the bill late into the spring (Herring Gull shows more pale color at the base of the bill and earlier in the season); an overall brown and white scalloped or checkered look to the body feathering (there is a great deal of variation in the overall tone--some birds are fairly pale, like the bird shown here, some are considerably darker overall); pale pink legs; and dark brown primaries with pale edgings outlining them.

Also see: A Comparison of Common Gull Heads in winter and breeding plumages