Yellow-legged Gull - Larus michahellis

(last update: October 08 2013)

Coordinators:
Delfín González
Gabriel Martín
Antonio Gutierrez
Amir Ben Dov
Mars Muusse

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Larus michahellis - 2CY April

This website deals with the Yellow-legged Gull taxon michahellis, which is a common migrant from July to December in NW Europe. After extensive expansion of the breeding population during the last three decades, it nowadays can be found breeding in Belgium, the Netherlands and Britain in mixed couples with both Herring Gull (argenteus) and Lesser Black-backed Gull (graellsii). There are subtle differences between the populations from the Mediterranean, Atlantic coast of Portugal and Morocco and from the islands in the Atlantic. Most pronounced differences can be found in the taxon atlantis, now regarded as full species by some authors and birds along the Atlantic coast of the Iberian peninsular: lusitanius. Both atlantis and lusitanius are treated in their own sections on this website.

General description:

By April the numbers of 2CY michahellis slightly increase at Etaples in NW France with a few 10's of birds in the direct surrounding of the fish-market. 

By mid-April, 2CY michahellis start dropping the inner primaries and the first wing-coverts. The feathers which were included in the post-juvenile moult are normally left out of this spring moult. However, the exact figures are hard to obtain, as birds which do drop these earlier moulted feathers are not recognized of course (unless ringed and reported several times). 
The general appearance of 2CY michahellis is rather straightforward in 95% of the birds. Strong and powerful males are easily recognized between 2CY argenteus. An example of the average michahellis can be found in the first and fourth image. Birds showing a combination of grey adult-like lower upper scapulars, a pale head contrasting with the dark neck-collar, dark centred tertials and wing-coverts moulted in the post-juvenile moult are very likely to belong to the michahellis taxon and such a combination of characteristics would be very odd for argenteus by mid-April. In the tenth and second image 2CY argenteus can be compared to two 2CY michahellis on the bank of the small river Canche at Etaples (more information on this excellent site for studying michahellis can be found on the Le Portel Page). Image 2 is illustrative for the differences in argenteus and michahellis in this time of the year with the very obvious bold pattern in the lower scapulars in michahellis contra the rather plain lower scapulars in argenteus. Note however that argenteus Herring Gulls may show a wide variation of patterning in the lower scapulars, including some individuals showing similar bold patterns. Check out the 2CY argenteus images in the April Section and the May Section. In most birds, and especially when direct comparison is possible with argenteus, michahellis proofs to be ahead in moult. Nonetheless, michahellis may show overlap with argenteus from southern populations (argenteus breeds in the industrial area of Boulogne/Mer). Remarkably, michahellis may not be the most advanced moulting species. On average, first summer Great Black-backed Gulls at Boulogne/Mer may be even further ahead in moult than michahellis.

The first image represents strong male michahellis. Another distinct 'group' of 2CY michahellis can be recognized and are represented by the bird in image 6. These birds are similar in jizz and size as the first 'group' and are easily picked out of a group of argenteus Herring Gulls as well. They do not show third generation grey adult-like scapulars but have the third generation scapulars with barring or anchor patterning. The base colour is often brown-buffish and the pattern, which can be bold and obvious, is normally repeated on the wing-coverts and create in a complete barred plumage in freshly moulted birds (although this completely moulted plumage can not be found in April birds and is only obtained by June. See the June Section for images). The bird in image 6 is exceptionally advance in this moult, although it must be kept in mind that the majority of the wing-coverts were largely replaced in early winter during the partial autumn (post-juvenile) moult.

Primary and rectrices moult stage and pace:

What's the average moult in April? We described 19 2CY michahellis in detail and the following pattern can be extracted:

By mid-April, the complete moult is just starting with dropping the first inner primary, with the majority of 2CY birds (> 60%) still showing juvenile p1-p10. About 30% of the 2CY michahellis have started dropping p1 and exceptionally p2.

The majority (> 80%) of 2CY michahellis have all the rectrices still juvenile. A minor 15% show second generation tail-feathers. These second generation rectrices may either originate from the partial autumn moult, while other birds just start the moult of tail-feathers by mid-April. 

Wing-covert moult stage and pace:

In several sections of 1CY michahellis it's mentioned that birds include wing-coverts in the post-juvenile moult (unlike argentatus / argenteus where this post-juvenile moult is confined to the scapulars in 98% of the birds). To distinguish between 2CY birds which have still suspended moult (after the post-juvenile moult) and birds which have really started to replace wing-coverts, we made a separation between "old second generation" feathers and "fresh second generation" feathers. The results are presented in the table below. 
Except the difference in timing of moult, these second generation feathers may show a different pattern as well, with fresh second generation feathers moulted by April looking 'less immature', i.e. the base colour of the feathers is not buff-brown but buff-grey and the pattern may be less well-defined.
The table below present the figures of active wing-covert moult by April: although many birds show second generation wing-coverts (especially in the lesser coverts), only very limited number of birds acquired these feathers by the complete moult which started this month. Most second generation wing-coverts were already acquired by the post-juvenile moult last autumn - early winter.

Mantle and scapular moult stage and pace:

Replacement of scapulars follows a highly standard sequence in immature michahellis. Early after fledging, by August, the first juvenile scapulars are dropped in the lower row of upper scapulars. Subsequently, scapulars higher on the back are dropped, together with the first mantle feathers. Once most of the upper scapulars are replaced for second generation feathers, 1CY birds start dropping lower scapulars; it's September then. By the end of September, most of the upper lower scapulars have been replaced. By October, the lowest row of lower scapulars will be replaced, starting with the inner feathers. By the end of October, it's only the longest rear lower scapulars, the once covering the tertials, which may still be juvenile. These rear lower scapulars are moulted in November, so by December most birds show a complete second generation scapular and mantle region. Michahellis, being a rapid moulting species, may almost immediately continue replacing the lower upper scapulars again, now to third generation. The second generation feathers in the lower upper scapular row are already half a year old, may be very worn at the fringes and bleached. By April, this moult to third generation is well under-way.

Moult to third generation scapulars has started in all birds, although many birds have only replaced a few lower upper scapulars. Nevertheless, two of the 19 birds present at Etaples, NW France (app 10%) have over 50% of the scapulars replaced for third generation feathers. 
Advanced moult in the scapulars doesn't necessarily come along with advanced moult in wing-coverts. One bird, falling in the 50-75% class of grey third generation mantle and scapulars, had all wing-coverts, tertials, rectrices and remiges still juvenile. 

Some 2CY michahellis may show a red orbital ring by mid-April.

Tables: