Great Black-backed Gull (Larus marinus)

(last update: October 12, 2011)

Great Black-backed Gull 2cy January

Plumage sequences in 1cy and 2cy GBBG

The vast majority of Great Black-backed Gull follow a simple, classic moult sequence and moult timing, when compared to other large white-headed gull taxa in Europe. In this respect, it very much resembles Herring Gull. Both Herring and Great Black-backed Gull are nice species to monitor moult, as birds in these taxa replace (most of) the wing-coverts and tertials only once a year.

Only a very small fraction of 1cy Herring and Great Black-backed Gulls (about 2%) replace single coverts in the first autumn, but these exception birds can best be skipped to get a good picture of the normal moult sequences.

plumage 1cy August - 2cy April
The juvenile plumage in GBBG is largely attained until 2cy May; of course scapulars are replaced, but no replacement of coverts and tertials in the post-juvenile moult (autumn 1cy GBBG). This is different from taxa like graellsii, cachinnans and michahellis, where many individuals include the upper tertials and inner wing-coverts in a post-juvenile moult (see 5 example birds below).

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The extent of post-juvenile moult in 5 taxa. Graellsii, michahellis and cachinnans often replace some wing-coverts and upper tertials for 2nd gen feathers. Marinus and argentatus/argenteus keep juvenile wing-coverts and tertials until April. Images are links: Larus fuscus graellsii (ringed in NW France) 2cy spring, February 12 2007, Agadir, Morocco. Larus michahellis 2cy spring, February 19 2012, Zagreb, Croatia. Larus cachinnans 2cy spring, February 19 2012, Zagreb, Croatia. Larus marinus (ringed in N Russia) 2cy spring February 08 2011, Scheveningen, the Netherlands. Larus argentatus argenteus (ringed in the Netherlands) 2cy spring, February 20 2011, IJmuiden, the Netherlands.

Throughout the first winter period, marinus and argentatus keep a juvenile wing-covert panel that start to show wear at the fringes in spring. In these taxa, the first new coverts appear in early summer of the 2nd calendar year; covert moult is from May to late August.

plumage 2cy May - 2cy September
In summer all Herring and Great Black-backed Gulls renew all the wing-coverts and tertials for the first time. Most (and, in some birds, all) of these 2nd gen coverts are kept for the next 12 months. This wing-covert moult completely overlaps in time with the complete moult in summer, which starts when P1 is dropped in late-April or early May.
Of course, not all wing-coverts and tertials are shed simultaneously. Moult is initiated at certain loci, and then progresses throughout the entire wing-covert panel. To completely replace the whole wing-covert panel, it takes about 4 months.

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The progress of complete moult in coverts and tertials, in 5 stadia. 1. Normally, the first wing-coverts to be dropped are innermost greater coverts, innermost median coverts and innermost lower lesser coverts. Then the process speeds up and some birds may have the complete set of central median coverts missing (see link 1). 2. As the median coverts were dropped almost simultaneously, new feathers grow in as groups creating a complete fresh row (see link 2). Fresh feathers always have a buffish brown or greyish base colour, which only start to bleach to whitish after 4 or 5 weeks in the sun. Now the central feathers are new, the outer feathers are dropped. Most greater coverts and lesser coverts remain old juvenile. 3. Median, lower lesser and many lesser coverts are replaced and equally new. The outer greater coverts are dropped now. The last group of juvenile wing-coverts are located at GC6 - GC10 and lesser coverts in the carpal edge (see link 3). 4. From the outer region, one moult wave progresses inwards in the greater covert row. Another moult wave progresses at slow pace from the inner greater coverts. The 2 waves will meet at GC7-GC9, which are the last central greater coverts to be dropped, exposing the bases of the secondaries underneath (see link 4). In this phase, the central tertials are replaced, the lower tertials are still juvenile. 5. The moult gap in the greater coverts closes, the last feathers obvious buffish in base tone again. Coverts which were included in the start of the moult process are already bleached by the summer sun, now whitish based. The very last coverts to be replaced are small feathers in the carpal edge, but this may be difficult to detect. The wing-covert panel is now completely 2nd generation.

The 5 images above illustrate the process of the complete moult in the wing-coverts in the summer months. By August the wing-covert panel and tertials completely exists of 2nd gen feathers. In many birds this does not last very long, as 2cy autumn birds moult a limited set of wing-coverts and again the upper tertials; these feathers then turn into 3rd generation.

plumage 2cy September - 2cy November
By September we often see an extra moult in wing-coverts, which we call a "partial autumn moult" at this website. A partial autumn moult is seen in many white-headed gull species (graellsii, intermedius, michahellis) but Great Black-backed and Herring Gull are best to show what happens. We use "partial" in the definition, as it really is a very limited, yet common process.
For marinus and for argentatus (especially argenteus, far less in nominate argentatus), about 2/3 of the birds have a partial autumn moult in September - November. A limited number of feathers are replaced again, now turning into 3rd generation.
Included are upper tertials, innermost greater coverts, quite some median coverts (sometimes all visible MC) and randomly single lower lesser coverts and lesser coverts. It may be not a surprise that these are exactly the coverts which were replaced early in the complete moult process.
When scoring birds on partial autumn moult, you can fix on specific groups; e.g. in the greater coverts, only the very innermost feathers may be included, at most 4 or 5 feathers. The central and outer greater coverts always remain 2nd generation; only moulted once (late June) and not replaced again before June 3cy.

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The partial autumn moult in 2cy GBBG. 1. & 2. 48C 2cy & 3cy, September 02 2008 and March 12 2009, Boulogne sur Mer, NW France. As 2cy in September, moult in coverts and tertials was finished. So how comes we see this bird back in March with many dark median coverts and inner greater coverts? Because a partial autmn moult took place. (see link 1 & 2). 3. Active moult gap in the inner median coverts by late-September; this must be a strong indication for partial autumn moult. Indeed, also inner greater coverts are fresh. (see link 3). 4. Bird from Rogaland, Norway. Complete row of visible median coverts replaced for 3rd gen. Pattern much resembling 2nd gen feathers. (see link 4). 5. Male from Denmark. Inner Gc, some MC and some LLC replaced for 3rd gen in partial autumn moult (see link 5).

The pattern on these new 3rd generation feathers is highly variable. Often they very much resemble 2nd gen feathers, and if so, the only way to tell these feathers are 3rd gen, is by the buffish or greyish base. In other birds, the new 3rd gen feathers may appear more adult-like.

Central and outer greater coverts, and most of the lesser coverts as well, are only replaced one time in the annual cycle, they are never include in the partial autumn moult. Probably a very similar partial autumn moult can be found in all white-headed gull taxa, not only in marinus and argenteus. The problem describing such processes accurately lies in the extent of the post-juvenile moult. Marinus and argenteus do not include tertials and coverts in the post-juvenile moult, so we can very much assume where the complete moult starts, and we can easily tell that inner greater coverts and inner median coverts are replaced twice in their 2nd calendar year. But michahellis, graellsii and other taxa have highly variable post-juvenile moult in the wing-coverts, and subsequently it is much more difficult to tell exactly what is going on in summer and autumn. (Is this bird in July replacing the old juvenile feathers, or is it also replacing old 2nd gen feathers from the post-juvenile moult? Hence, will new feathers be 2nd gen or 3rd gen feathers? You just don't know. Only long periods of observations on ringed birds will lead to satisfying conclusions.)

Moult timing in first winter birds: January

Birds in the North Sea bassin and along the coasts of the Channel probably reached their winter grounds. Such birds already appear as early as August. Gradually it becomes colder in N Scandinavia and late hatched juveniles arrive as well after the first cold nights in November and December. This may explain why we still see birds with complete juvenile scapular region in January.

Tables:

below: graph corresponding to data from table 1. ringed birds n=10, unringed birds n=19. Distribution of 2nd generation scapulars in January 2cy GBBG, separated into Upper Scapulars, Lower scapulars and for Total scapular region. Classification in 6 classes, running from 0% (no 2nd gen scaps) to 100% (all scaps in a region are 2nd gen).

Suddenly, in the column "Total scapulars" one ringed bird appears scoring "0% 2nd gen scaps", in other words, in January JH752 from far N Norway was still completely juvenile. When unringed birds are scored as well, such completely juvenile birds are encountered more often.

Below: progress in scapular moult in 1cy & 2cy L. marinus from August to April.
The group of birds which score 100% 2nd gen scapulars gradually grow; remember however that 2cy birds, especially in March-April, grown fresh upper scapulars again, most probably 3rd gen feathers. Figures in the graph are not adjusted for this; and the graph may be read reverse, the presence of juvenile feathers dimish through time.