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 - adult August

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

August is one of the best months to study michahellis at Le Portel and Etaples, NW France. At the beach of Le Touquet, near Etaples, large groups congregate and use the beach as a resting place. They frequent the Dannes landfill as feeding place and many depend on the local fish-market of Etaples for food supply. The ports of Boulogne-sur-Mer and the beach of Le Portel are other good spots to find adult Yellow-legged Gulls. See Le Portel page for locations.

The difference between sub-adult and adult birds is sometimes very difficult. We used the plain grey greater primary coverts as criteria in our scores. For details on differences between michahellis and cachinnans, see e.g. the article by R. Klein & D. Gruber in Limicola, April 1997.

Moult of remiges and rectrices in August

This is the last month research on remaining old primary scores can be executed, as the group of birds with all primaries shed still does not influence the mean and SD-value significantly. The weighted mean value for the end of August at Etaples and Le Portel is 1.59, with n = 133 and SD = 0.69. Again the narrow range of remaining primaries is obvious, expressed by the low SD value. As the table shows, the average remaining old primary score is 1.6, with still one week to go in August. Extrapolation will result in a remaining primary score of about 1.5 for August 31. By the end of July this score was still 2.35 with n = 106 and SD = 0.63. 
Linear regression analysis of first, provisional data collected for Yellow-legged Gull, Lesser Black-backed Gull and Herring Gull (data from NW France and SW Netherlands) show some differences between the three species. Michahellis is an early moulting species, as can be expected as it starts breeding relatively early. It is on average about a full primary ahead of the two other species. Furthermore, compared to argenteus Herring Gulls, it also is somewhat quicker in moulting the primaries (shedding primaries at a higher pace) and therefore the figure shows a steeper regression line (larger regression coefficient in the formula). In future, pages on these differences will be added on his site.
The secondaries are shed from the first week of August onwards. The outermost secondaries (about S1-S4 are shed first and at the time S1 and S2 start growing, the central secondaries are shed quickly one after another in the first weeks of September. Compared to 2CY michahellis, adults are about one and a half month delayed in secondary moult, but the sequence is similar. In the inner-arm, close to the tertials, a small block of about four feathers remain unshed, probably to guarantee a certain level of maneuverability. The innermost secondaries are shed in the same period as the central secondaries. 
Rectrices moult is in progress throughout the month with complete new feathers visible at the end of August.

Moult in the wing-coverts

Moult in the wing-coverts is very obvious in adult michahellis in NW France by early August. A large amount of the (central) greater coverts and median coverts are shed and some of the secondaries are visible almost to the base, even when the wing is folded. Simultaneous moult starts in the central tertials.

In the scapulars, the upper scaps and upper lower scaps have been replaced; moult progresses in the lowest row of scapulars. Moult in the scapulars is limited. The mantle and scapular region is moulted throughout the summer period, but you hardly find large moult gaps of adjacent missing feathers in these tracts. This seems reasonable as most energy has to be put in replacement of the remiges, rectrices and coverts. Nevertheless, the lowest row of lower scapulars (lls, see the Gull Topography Section) may show missing feathers, especially in the central lls and in the rear lls. Randomly, some upper scapulars may be included in moult as well but often only at a slow pace, one-by-one. The outer lls may appear very worn by August.

Adult michahellis may include the upper two tertials in the partial moult in autumn. This may explain the moult pattern in some adults by the end of August. The majority of adult michahellis drop the 3^rd and 4^th tertial, while #5-6 look very abraded. The tips of the upper two tertials show wear as well but are not as abraded as the lowest tertials. The moult sequence in August normally progresses from the 3^rd tertials downwards. Some individuals may continue to include the upper two tertials again in the partial moult of October.

The average adult michahellis in August has a few innermost greater coverts replaced by new feathers and has growing new feathers from the outer-wing inwards, resulting in a large moult gap in the central greater coverts. The moult sequence in the greater covert bar is similar to that of e.g. Lesser Black-backed Gull and Herring Gull. The inner three or four greater coverts are shed first and are growing at the time another moult wave is initiated at the outer greater coverts. Subsequently, the greater coverts are growing in two moult waves: one from the outer-wing inwards and one from the inner greater coverts outwards. The two waves meet at greater covert #7 or #8. By the time the two moult waves meet, the inner greater coverts may show slight wear at the tips already. This is the common moult sequence as can be found from late July to October, but other sequences may be shown as well, including a quite random "jumpy" sequence in the greater coverts, with every other one replaced.

The median covert moult is slightly ahead of the greater covert moult and by the end of August adult michahellis close the moult gap in the central median covert. This can for instance be seen in a sequence like in image 8: The inner five median coverts are fully grown, #6 is slightly shorter and has to grow an inches or so. Another moult wave is growing from the outer-wing inwards, with the last fully grown feather at about #11, while #10 falls a little short, #9 even shorter and #7 just visible. It's obvious that the two moult waves will meet at #6-#7.
The lower lesser coverts (llc) normally follow the same moult sequence as the median coverts, but are slightly behind in time. 
The lesser coverts are moulted more randomly and especially the outer lesser coverts (the carpal edge) may show abraded remaining coverts throughout this month. The wing-coverts in the carpal edge are the last replaced coverts, at the end of the basic moult.

Timing and strategy of body & covert moult in August

The winter head becomes more obvious in August where streaking is concentrated around the eye, on the ear coverts and to the nape, sometimes running down in the neck. Moult of the flank feathers was started already at the end of July. By August, feathers in the breast and vent are moulted as well. Some adult still show head moult around the eye (e.g. image 4).

See the Topography Section for explanation of feather tracts.

Naked parts

Some birds develop black markings on both the upper and lower mandible by the end of the month.

Tables