Last update of this page: 25-07-05 18:30

In this section we include LBBG in 4cy and 5cy. Some very hard to age 3cy may be presented as well. 

Movements and origin

By October, all LBBG have left the colonies and surrounding feeding grounds near the Maasvlakte (the Netherlands) and migrate south towards France, Portugal and Morocco. In November and to a lesser extend December, a couple of hundreds remain on continental NW Europe, as north as the south of the Netherlands along the coast at Westkapelle, 65 km south of the Maasvlakte (see map). As it seems, every year a larger group of LBBGs stay "at the most northern latitude as possible", i.e. migrate south only when winter conditions force them to.
Ringed individuals can be found until late November (e.g. year 2000, a rather mild winter until mid-December) and they originate from Scandinavia, Denmark, the Netherlands, Belgium and the U.K. When winter really starts in December, these groups move south to unknown wintering grounds, probably the Atlantic coast of West and South-West Europe.

Identification of sub-adults

Sub-adult LBBG resemble birds in full adult plumage, especially in the grey upper-parts and all-white under-aprts. 4cy LBBG may show all grey wing-coverts and scapulars and, like full adults, may have a white mirror on the old P9 and P10 and a relatively low primary moult score. Normally, 4cy birds have the tail all white by October. However, most sub-adults show immature features to various extends, with sometimes some lesser coverts still immature (brown centred feathers in the carpal edge), extensive dark centres in the central tertials and sometimes some diffuse patterns on the greater coverts. Strongest clues for immaturity are the very well-developed winter-hood in some birds, the well-developed black bill-band and immature coloration of soft parts: bill, iris and legs. Note however that many sub-adult birds have moulted wing-coverts and have shed the outer primaries, becoming similar to birds in adult plumage. Best clues for identification are often found in the old feathers.

The difficulty in identification between 3cy and 4cy birds is nicely illustrated by a 3cy intermedius LBBG ringed 4272452 Copenhagen. It's advanced in upper-part coloration, but note also that P8 is slightly longer than P7, while P9-P10 are still old; moult will probably be arrested at P8. This bird was ringed on July 07 1999 at Langli Ho Bugt Vadehavsoerne, Western Denmark (55.31N,08.19E).

Moult stage and strategy in sub-adults

The complete moult of sub-adults follows the timing and strategy of adult birds, although this timing is a few weeks earlier. By October, the overall picture is somewhat clouded by the mixed flocks of birds originating from British graellsii colonies to Swedish intermedius colonies. By clicking the first thumbnail, you'll find three birds from the Dutch Maasvlakte colony with a very similar moult pattern. 

The complete moult in these three birds has almost been completed with the median and greater coverts replaced. The upper tertials are replaced as well but the lower tertials and the inner secondaries are still old. The new feathers appear shiny dark grey. The old feathers are brown bleached and worn at the tips. The outer primaries P9 and P10 may still be present but the more advanced birds have all the outer primaries shed and in these birds P9  may already be longer than P8. These advanced birds have the secondary moult completed as well. The tail is new and completely white in almost all birds and may show a little wear in the tips already in some birds.

Arrested moult in  immature LBBG

Some sub-adult LBBG may arrest or suspend the primary moult in autumn. Most observations of LBBG with arrested or suspended moult in continental NW Europe apply to immature birds, both in spring and in autumn. The bottom four images show birds with arrested moult. 

One may wonder about the extend of this phenomenon in intermedius and graellsii LBBG. Peter Stewart, who caught many adult LBBG in Britain found quite some birds with arrested moult in summer and we assume adults may arrest the moult for the high energy demand and efforts in defending territories and raising young in summer.

Peter Stewart comments on arrested moult in LBBG: "You have written that arrested moult is a rare phenomenon in both graellsii and intermedius. This is incorrect. It is quite normal, and here in the UK suspended moult is obvious in many adult birds we catch, mainly from June to July. These birds replace only a single primary at a time and can be found with up to suspended P3. Many other birds caught during the same period are about to suspend or resuming moult after suspension by dropping the next primary. We have also found adults birds in April with suspended P1, and these birds are obviously commencing primary moult and suspending in winter quarters, as we have yet to find a bird that has started primary moult in February and March in the UK."
"Suspended moult, especially involving inner primaries, is not obvious in the field and easily overlooked, especially on birds that show little wear on old primaries. Most earlier researchers have overlooked suspended moult, hence the belief that some birds have yet to start their primary moult, and provide later starting dates for the onset of moult. I have some 4500 moult scores for adults, and recorded 183 different primary moult patterns for adults, with a total of 201 different patterns for all age groups. Sick and injured birds in captivity, can also behave differently and can also suspend at odd times." More about Peter's research can be read here. 

Except that birds may be ill or injured, another reason for autumn birds arresting the moult is migration. It's highly inefficient to migrate over a long distance with a gap in the wing. For this long migration, the inner primaries seem to be of high importance, and that may support the general picture observed in the field: if possible, at least the inner primaries are moulted before departure takes place. Of course, the next very good reason is lack of time to fully complete primary moult, but in sub-adult birds, this could be avoid easily by an earlier commencement of the primary moult. M. Hario, doing research on fuscus LBBG in S Finland, scored primary moult in adult birds. Breeding success was very low in his research area, leaving the adult with a lot of "surplus time" and "surplus energy" now the nests were destroyed or the young died. Nevertheless, most birds remained to develop the original moult strategy and timing. More about Hario's research can be read on this adult fuscus page.

Arrested moult in  immature LBBG