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 - 1CY September

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 September, the vast majority of 1CY michahellis has rather worn fringes at the juvenile feathers and the brown centres are already somewhat bleached. By this month, most of the mantle feathers and upper scapulars have been moulted more than a month ago and the buffish / gingery centres of the second generation feathers start to turn paler, almost white. Hence, these scapulars show almost white based centres, strongly contrasting with the dark anchor or barred pattern on the lower and upper scapulars. 

The majority of juvenile W European Herring Gulls and Lesser Black-back-backed Gulls have left the colonies as well and many of them appear slightly worn; resembling Yellow-legged Gulls. The clues to separate resting juvenile michahellis and graellsii become less reliable, but still, best characteristics in September are the inner primary pattern, tail-pattern and condition / moult-stage of second generation tertials and coverts.

Moult stage and pace of the scapulars

Throughout September, the mantle and scapulars are almost completely replaced by second generation feathers. Below, a table is presented with scapular moult scores of 20 juvenile michahellis in the Calais region, NW France. The vast majority of the birds have the upper scapulars moulted to second generation feathers, with only a few smaller upper scapulars remaining juvenile (therefore a moult score of the fully 100% only occurs in a few individuals, but within the upper scapular moult-class 75-100%, most birds have moulted over 90% of the visible upper scapulars). 

By the end of the month September, a few moult gaps and single retarded juvenile feathers may still be present in the outer upper scapulars. The clearly visible lower scapulars, which are moulted later than the upper scapulars, may still show the brownish or buffish base between the dark bars or anchor pattern in the second generation feathers. 

As can be seen in the images, these fresh second generation feathers have a broad white fringe. The feathers which were moulted longer ago have the base turned paler and show contrasting brown/black - white feathers. Some second generation scapulars already show wear in the fringes by September. Those feathers can be found in the rear upper scapulars and the rear upper lower scapulars. 

By the end of the month, there may still be moult gaps in the lower scapulars, especially the central and rear feathers in the lowest row of lower scapulars (the feathers covering the scapular coverts). Moult scores in the first week of October reveals that the rear feathers in the lowest row of lower scapulars are still present and juvenile in the majority of the 1CY michahellis.

Moult stage and pace of the wing-coverts and tertials

The covert moult stage and tertial moult stage may vary to a large extend. Some birds still show a complete juvenile wing, with worn covert fringes (especially the inner coverts) and juvenile tertials. These 1CY michahellis birds strongly resemble graellsii when the wings are folded and the bird rests. By the end of the month, about 25% of the 1CY michahellis have all the wing-coverts and tertials still juvenile. Some of these juvenile birds may still start the partial moult by late September, as the figures slightly drop by October. Wing-covert scores from late October show that only 15-20% of the juvenile birds still have all the wing-coverts juvenile. Over 80% have replaced at least a single wing-covert to second generation feathers.

Most 1CY birds show advanced wing-covert moult, normally including the inner medians, the inner lower lessers and the inner greater coverts. The wing-covert moult is often most advanced in the median covert row, which can consist of second generation coverts for over 50% (most of the visible median coverts in a folded wing). Less than 50% of 1CY birds start moulting the tertials in September, often starting with the upper two tertials or with the third tertial. The juvenile tertials are plain brown centred and the fringes are almost completely worn away, leaving a protruding shaft streak. By early October however, many birds shed the upper tertials and figures from late October show replaced upper tertials in the majority of the birds.

Replacement of the tiny lesser coverts is hard to establish in the field and most of the time impossible unless close range images are obtained or the bird is examined in the hand. In the table below, "missing lesser wing-coverts" must be read as "feathers missing or growing". Still, this doesn't fully eliminate the problem of judging the stage of moult in these feathers, as it may remain debatable whether a feather is fully grown or not.

Moult stage and pace of the rectrices

The small sample of birds in the table below didn't include any juvenile michahellis moulting the tail-feathers. A sample from the first week of October included several birds with moulted rectrices which confirms the general idea that at least a small minority of 1CY michahellis includes tail-feather replacement in the post-juvenile moult from October onwards. In any moderate group of first calendar year michahellis, birds can be found moulting the tail-feathers by October (see the October section for tables). The exact extend of this moult is still not known, but in spring, 2CY michahellis with second generation tail-feathers can be seen regularly (even birds with the tail completely moulted). Next year, more research will be executed on this topic.

Movements

The majority of michahellis in NW France arrive from SW origins, the colonies from Marseille and the Mediterranean coast towards NE Spain, Italy, Switzerland and the German Rhine colony. Some birds must have origins further south, as the ring recoveries of blue ringed Sardinian birds and black ringed NE Spanish birds indicate.

Atlantic Yellow-legged Gull atlantis 1CY, September 01 2010, São Miguel, Azores Islands. Picture: Richard Lowe. Most scapulars replaced (except lower row of LLS), now active moult in all covert tracts and upper tertials. Note also moult in head, neck and sides of the breast. Note typical brown aspect in Atlantic birds.

Yellow-legged Gull lusitanius 1CY, September 06 2009, Xove / Burela, Galicia, Spain. Picture Gabriel Martin. No tertials or wing-coverts replaced. Lower scapulars and several upper scapulars still juvenile. This 1CY bird show some features that can be considered "typical" for juvenile lusitanius: long wing-projection; piano-key pattern on the inner and central greater coverts created by the deep notching; rather plain tertials with thin pale fringes; weak gonydeal angle on the lower mandible resulting in not really a strong bill (particualrly in females). Elegant jizz, at least more elegant than the stocky Mediterranean michahellis, in combination with the late moult timing, make these lusitanius birds sometimes really hard to separate from from LBBG.

Yellow-legged Gull michahellis 1CY, September 15 2008, Mamaia, Romania. Picture: Chris Gibbins.  From east Med. population. About half of the scapulars replaced, coverts and tertials still juvenile, except for one inner MC.

Tables

Yellow-legged michahellis U2FJ 1CY, June 18 & September 28 2011, Israel. Pictures: Amir Ben Dov. U2FJ, U8BC, U7CF.