Extract 1. — from the introduction
In summer, many gulls look untidy owing to poor condition of the old plumage. When head-feathers are worn or moulting, the bill may look disproportionately large. If moult is delayed, most of the plumage often looks extremely bleached and untidy. Birds with extremely worn flight feathers fly with quicker, more laboured wing-beats than in normal conditions. First-summer Black-legged Kittiwakes show this wear and flight at its extreme, but ‘retarded’ large gulls may also look extremely bleached and weakened. During the moult of greater coverts, the unmoulted darker secondaries of younger gulls may be exposed, creating an impression of all-dark greater coverts. This has confused observers watching younger large gulls that lacked most greater coverts at one time, leaving the dark secondaries as a ‘dark greater covert bar’. Similarly, older dark-winged gulls which have simultaneously moulted all their greater coverts expose the pale bases to the secondaries, leaving a pale bar on the midwing (easily seen in, e.g., late summer black-backed gulls).
Features such as primary projection, position of wing-tip related to tail-tip or tertials and pattern of outerwing are useful in fully grown birds, but more or less useless between midsummer and late autumn, when the outerwing is growing: everyone who had tried to judge wing-tip position compared to tail-tip in large gull gatherings will have experienced this problem, e.g. Iceland Gull normally has a long wing projection towards the tail as a useful character to distinguish it from the shorter-winged Glaucous, but the lack of outer primaries in moulting Icelands has been known to frustrate observers (Hume 1980). Less well known is the fact that certain large gulls grow the whole tail at once, resulting in an unexpectedly short tail (and therefore seemingly extremely long wing-tip) in autumn. On the other hand, the exact pattern on certain outer primaries is much easier to assess when the bordering primary is missing. Preening birds often exhibit both upper and lower surfaces of the primaries well.
It should also be noted, that vagrant gulls may differ slightly from their normal moult pattern. This has been demonstrated in Franklin’s Gull in Europe and Australia (see that species). Southern Hemisphere gulls eventually settling in the Northern Hemisphere may adapt moult cycles similar to Northern Hemisphere gulls.
Extract 2. — from the introduction
mapIn gulls, males are larger with a heavier, deeper bill and
flatter head than females. The larger the species, the more obvious the difference is.
Also, first-years are smaller than older birds. Therefore, in a given species, a
first-year female may look unexpectedly small compared to an adult male (this difference
most obviously in the larger species). This complex situation is a main reason for the
inclusion of measurements of both adults and first-years in the species accounts. Try to
use these measurements when comparing gatherings of gulls, preferably of several species.
Only by direct comparison—best in pairs—is sexing advisable. While general
impression (jizz) and size, once learned, are very useful when identifying gulls (the jizz
of a male and a female differs), the specific differences in jizz between closely related
species are often overestimated and have led to some questionable identifications. For
example, head and bill shape in the large gull complex have been introduced as characters
separating Yellow-legged Gull from Herring Gull, and also within the Herring Gull
subspecies, where the use of these characters remains tentative. While certain individuals
(especially large adult males and small first-year females) do show genuine differences,
most may at first glance look identical. Be careful and patient and combine both jizz and
feather details before a rare species is claimed: few other bird groups are so variable—or
have so many individuals causing so many headaches among specialists—as the
fascinating but frustrating larger gulls. No-one will ever be able to identify every large
gull!