Saturday, 7 July 2012

Summer gulling on the Thames

Third-calendar-year Yellow-legged Gull: one of at least 32 in the Rainham/Crayford area of London yesterday.
While the gulling season proper is still some months away, there is good reason to be out looking at larids again right now. In midsummer every year, after dispersing from their breeding grounds further south in continental Europe, Yellow-legged Gulls Larus michahellis begin arriving in numbers in southern England, and moving up the Thames. In due course hundreds will be present; at Rainham last Saturday we noted four on the Crayford foreshore, whereas just six days later Kev Jarvis and I had at least 32 between us.

Adult Yellow-legged Gull yesterday: birds of this species are now actively renewing their plumage.
It's possible to see all ages of Yellow-legged Gull at this time of year, with the first, fresh juveniles arriving alongside adults, but both were outnumbered yesterday by one-, two- and three-year-old birds. What's more, unlike the winter season, it's a good opportunity to observe birds in active wing and tail moult. According to Olsen and Larsson (Gulls of Europe, Asia and North America, 2004), Yellow-legged Gull begins moulting "from a few weeks to three months earlier than in Herring Gull", with "P(3)5-6 late Jul/early Aug". Here's an annotated close-up from yesterday to show what's going on in adult primary moult right now:

A number of third-calendar-year (aka second-summer) michahellis were also present, with one providing another opportunity to look in more detail at wing moult in this age class:

Third-calendar-year Yellow-legged Gull yesterday and, below, the same bird's right wing-tip:
The timing of the moult is very similar to that of the adult above. Some (but not all) of this feather detail can be worked out on birds at rest, but in flight it's easier (NB this is a different 3cy bird below):

Third-calendar-year Yellow-legged Gull in flight, showing the obvious extent of primary moult: about halfway there.
Lastly, two young Yellow-legged Gulls were my first juvenile large gulls of any species in Britain this year. While Herring Gull can be ruled out on plumage, it's less easy to exclude Lesser Black-backed Gull. However, in terms of the slight barring they show on the inner greater coverts, as well as size, structure and general appearance, I feel pretty confident that both these birds are Yellow-legged. The early date is also a plus point for this more southerly (and thus earlier) breeding species.

Juvenile Yellow-legged Gull (bird A): this youngster has already suffered some tail damage. Note the darker, plainer outer greater coverts fading to a more barred pattern on the inners, the restricted primary 'window', slightly paler head, and largely white rump and uppertail with neat, sharply defined blackish tail band.
Juvenile Yellow-legged Gull (bird B): this individual is very similar to bird A above, but has subtly different
patterning on eg the greater coverts and underwing, as well as a complete tail.
Juvenile Yellow-legged Gull (bird B again): a very smart, fresh individual, as would be expected in early July. Note the tertial pattern, with narrow, unnotched pale edges which don't extend all the way along on the outer web (some are more 'oak-leafed' than this).

1 comment:

  1. Who says the gulling season hasn't started yet, Dominic? Excellent and instructive discussion, thank you.

    I'd like to pick up on one facet of what you've illustrated: your shot of the primary detail in adult YLG (above). This shows extensive white on P10, contrasting with virtually no white tips on the other unmoulted primaries.

    In my experience, it's quite common for a bit more abrasion to wear off the black tip to P10 and produce an extensive wholly white tip, which can tempt observers to suspect cachinnans.

    However, again in my experience, virtually all adult michahellis currently show almost no white on the tips of P6-9, resulting in an extensive area of almost solid black.

    By contrast, adult cachinnans can currently be expected to show obvious white on the tips of P6-9; certainly the case with the returning adult that's around in SW London at the moment.

    A small point, but I feel the P10 misconception can and does sometimes cause confusion, especially with adult michahellis that have dull yellow legs and apparently dark irides.

    Happy gulling,

    Rob Innes



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