The plumage of gulls is as hard to master as the conjugations of Latin verbs. Take the Herring Gull:
1st year, early summer: the chick is born with fluffy down. In summer she grows her first feathers; this pattern, called Juvenile plumage, is mottled dark gray across the wings and the mantle (the part of the back between the wings), lighter on head and rump, a dark bar across the end of the tail. Late summer and autumn: all feathers moult and are replaced by First Winter plumage: the gray across the wings has variegated so that it is darker along the rear edges and the tips.
2nd year, spring: head and body feathers moult, wing and tail feathers just fade so that the subtle variegation is increased; this is called First Summer plumage. Autumn: everything moults, to Second Winter plumage: across wings and back spreads the classic gull smooth blue-gray, but for the still black edgings and tips; white on head and body but for the remaining black bar on the tail.
3rd year, spring: again just head and body moult, while the wing feathers persist; this is Second Summer plumage. Autumn: all feathers moult, to Third Winter plumage: the gull-gray extends farther out along the wings (black retreating toward the tips), head temporarily goes back to gray, tail all white.
4th year, spring: head, body, and tail moult, all to white; this is Adult Summer plumage. Autumn: complete moult to Adult Winter plumage.
All the stages before Adult are called Immature. Having reached adulthood, the gull does not cease to change, but alternates between the two states already achieved, Adult Summer (by way of the partial moult of head and body each spring) and Adult Winter (by the complete moult each autumn).
That's just the Herring Gull. All others, too, moult partially each spring and fully each autumn, and the Iceland, Glaucous, Great Black-Backed, and Lesser Black-Backed Gulls do so as many times as the Herring Gull does before reaching adulthood. The others differ by having fewer stages: the Common, Little, and Mediterranean Gulls reach Adulthood one year earlier; Black-Headed and Sabine's and Ring-Billed Gulls and Kittiwake, two years earlier. And for all, the details of white and black and gray parts in each Juvenile and Summer and Winter Plumage differ. The heads of Black-Headed Gulls are not black but dark chocolate-brown, and in winter white with slight smudges on the sides.
Gulls are among the longest-living birds; at least, one Black-Headed Gull was found (having been ringed) to have lived 26 years and 3 months, apparently the highest for any British bird.
I hadn't realized there are so many kinds of gull. They must have been hard to sort into species: the Herring Gull, Larus argentatus, has a British race argenteus and a Scandinavian race argentatus; and its northern members are confusable with the Glaucous and Iceland gulls; another race is now regarded as a separate species, the Yellow-Legged Gull, Larus cachinnans. We try to divide living things into distinct compartments (and there are still people who believe they were created that way by God). There are many examples to show the difficulty of this, and the gulls provide a fine one, as described in a little book by Louis Halle called The Appreciation of Birds (1989, pages 19-21):
The herring gull and the lesser black-backed gull, Larus argentatus and Larus fuscus, are distinguished thus because in Britain they are clearly different species. But as you travel around the globe the characteristics of the one gradually morph into those of the other, the feet changing from pink to yellow and so on, until it becomes inescapable that this is one set, whose two extremes have met. "We may suppose [a characteristic Halle phrase] that an ancestral species occurred in the Bering Sea some twelve thousand years ago . . . its population spread eastward across North America and the Atlantic, and westward across Asia and Europe, until [they] met along the shores of Europe and Africa." Unlike the case of Homo, the intermediate links have not died out. So are they one species or two? It's an unanswerable question.
A blustery wind from the southwest (as soon as you step past the
corner of the Bell Cliff house you can feel it): the dispersed crowd
of thirty or so gulls almost stands in the air above the space in
front of you, all of them facing into the wind, all of them apparently
content to keep adjusting their bodies so as to stand there.