Comments on Identification of Certain Adult Gulls of the Pacific Northwest
This handout deals with gull species with white heads and white tails in breeding plumage that regularly (annually) occur in the Pacific Northwest. It is to be used with the Identification Chart for Pacific Northwest Gulls (breeding adults) and Measurements of Pacific Northwest Gulls.
Illustrations in all field guides previous to the National Geographic Society's Field Guide to the Birds of North America are quite inadequate for gull identification (Birds of North America is the worst), but fortunately the excellent paintings in the NGS guide by Thomas Schultz are accurate and useful. In addition, the photographs in The Audubon Society Master Guide to Birding, although not showing all plumages, provide a good supplement to the NGS guide. P. J. Grant's Gulls: a guide to identification, second edition 1986, is the best reference.
Size and shape variation
The gulls of our region vary greatly in size, in this group from Glaucous down to Black-legged Kittiwake. Within each species there is a moderate amount of size variation, and males are distinctly larger than females; this difference can be seen when a mated pair is examined. The bills especially are larger in males than females, very apparent in all the larger species, from California up. Both Glaucous and California vary in size geographically, which further complicates the use of size as an identification character.
Smaller species have relatively longer wings, which project farther beyond the tail tip and make them look more slender and "pointed" at the rear. This is an excellent attribute for assigning a perched bird to a size category.
Larger gulls have relatively broader wings than smaller ones, apparent as they fly overhead. As in most groups of birds, the larger species have slower wing beats than the smaller ones, again possibly of value when attempting to place a distant bird in a size category. Flight may be quite different under different wind conditions, so comparisons are safer than single birds. Black-legged Kittiwake has a peculiar rapid, choppy wing beat that immediately allows identification after gaining familiarity with it. Heermann's flies with rapid wing beat and wings strongly bowed, also a unique attribute. The smallest gulls (not discussed here) can be confused with terns at a distance more easily than with large gulls, because of their light, graceful flight and rapid wing beats.
The evenly colored upperparts in a gull, including the back and most of the wing surface, is called the mantle. Note that among these "gray-backed" birds there is considerable variation, with each species falling in a category from very dark gray to pale gray. The common species range from the rather dark Western through California, Mew, Glaucous-winged, and Black-legged Kittiwake to the rather pale Herring, Thayer's, and Ring-billed and the very pale Glaucous. Each species varies slightly, and the numerous hybrids between Western and Glaucous-winged range across the degree of darkness between the two species.
This appears to vary directly with darkness of mantle color, so species with mantles of different darkness can be separated by the underwing color when flying together (and therefore under similar lighting conditions). Paler species such as Ring-billed may look white under the wing, whereas dark species such as Western always show a dark shadow, usually darker toward the rear, under the wing. Backlighting will make any underwing look darker. Black-legged Kittiwake have a particularly noticeable white underwing at a distance, which belies the fact that their mantle is as dark as that of a Mew. Birds with darker underwings seem to show a more prominent white rear edge on the wing, as darker underwing colors contrast more than lighter ones with the white tips to the secondaries that all species have.
Wingtip color and pattern
Black-wing-tipped species usually have white spots at the tips of the outermost primaries, which may eventually wear off, and lighter areas ("mirrors") basal to the black tips. The amount of white in the wingtip varies within species, confusing the issue, but generalities are possible. Some individuals can look essentially adult but retain subadult primaries, in which the dark (black or gray) color is more extensive and the white tip spots are lacking.
Glaucous has pure white wingtips. Glaucous-winged has gray wingtips, in the lightest birds just a bit darker than the mantle. Perhaps because hybridization with Western is commonplace, some apparent Glaucous-winged have substantially darker gray wingtips than others. Glacous-winged X Western hybrids do not have wingtips sharply contrasting with mantle color, as the mantle and wingtips seem to vary in darkness similarly in a given bird. Note that Glaucous-winged and Herring also hybridize, with odd wingtip/mantle combinations produced; these hybrids may occur regularly in the Northwest.
Western has black wingtips that do not contrast greatly with the dark gray mantle. Herring and Thayer's have vivid black wingtips above, contrasting sharply with the pale gray mantle, the black not very extensive in either species. Herring wingtip looks the same above and below, but Thayer's wingtip is largely gray below (some individuals with scattered black spots), much like Glaucous-winged, and the contrast between black upper and gray lower surfaces of the wingtip is diagnostic of that species. Unfortunately, Herring and Thayer's just about overlap. Thayer's is as variable as any of our species in amount of black in the wingtip, may show very little above but always some. It usually has long, white mirrors contacting the white tip spots on one or more primaries.
California and Ring-billed have contrasty black wingtips more extensive than in Herring or Thayer's, California averaging slightly more black than Ring-billed. Ring-billed's extensive black wingtips and very light mantle produce a very contrasty bird. Ring-billed during fall molt can show very little black in wingtip and minimal white spotting (because of missing outer primaries), superficially just like Black-legged Kittiwake. Kittiwake always shows sharply contrasty black tip cut straight across, with no white spots tipping outermost primaries. Mew has relatively slight amount of black but much white in wingtip, conspicuous band of it crossing tip distinctive for long distance; may be as much white as black in some individuals.
Larger gulls have relatively thicker bills, an important aid in judging size, especially where comparisons are not possible. Thayer's is most easily picked out by bill shape, distinctly more delicate-billed than Herring, although body bulk only slightly less.
Stays the same all year in adults, although brightens in breeding season in some species. In this group, the yellow varies from quite dull in Mew to very bright, almost orange-yellow in Western, especially during breeding season. Western is a bit brighter than Glaucous-winged, Herring a bit brighter than Thayer's. The red spot on California enlarges in breeding season and obscures the adjacent black spot (thus eliminating the distinctiveness of the bill of this species), and some Californias have been seen with bright orange bills at this season.
Occasional individuals of other species, for example Glaucous-winged, have been seen with bright orange bills in winter.
Caution: All of the larger gulls with red bill spots, as their bill changes color with maturity, go through a "ring-billed" stage, when there is a poorly to well-defined dark ring near the tip, and a "California" stage, when the dark color has become restricted to the lower mandible adjacent to the developing red spot. Gulls that otherwise look fully adult can still have these intermediate bills.
Stays the same all year in adults; may be slightly more intense in breeding season. Intensity varies in each species from dull to bright pink or yellow. Thayer's slightly darker, more purplish-pink in some individuals, than other pink-legged species. Mew usually dull yellow, Ring-billed bright yellow. Variation greatest in California, from gray-green to bluish-green to yellow-green to yellow-orange in some individuals. Normally pink-legged Westerns also may have orange legs in breeding season.
Western and Thayer's typically vary from brownish to yellowish, although brown eye color is supposed to be diagnostic of latter species. Some Glaucous-winged have yellowish eyes, perhaps because of Western or even Herring genetic influence. Herring X Glaucous-winged hybrids may look like Herring but with brownish eyes. The variation in eye color in the large pink-legged gulls may all be attributable to hybridization. Birds with pale yellow eyes (Herring and Ring-billed) have a "cold" stare, those with brown eyes a warmer look.
Usually less intense and less obvious in winter. Difficult to see except at moderately close range but is an important difference among some of the larger gulls (the same group in which eye color varies).
Winter head markings
The extent and intensity of head streaking in winter is somewhat variable. Most Westerns have unstreaked heads in winter, a good field mark for that species from October to February, when the other large gulls start to molt. Head streaks on apparent Westerns may indicate hybridization with Glaucous-winged. Glaucous, Herring, California, and Ring-billed more likely to have streaky head pattern, Glaucous-winged and Mew mottled. The hindneck collar on Black-legged Kittiwake is diagnostic.