Last update of this page: 31-01-05 09:46

Regarding moult pace and strategy, sub-adults don't differ much from full adults. Sometimes it's hard to age sub-adults correct, but on average, some or all of the following features can be found in 4cy:

• relatively pale bare part coloration compared to full adults, although there is hardly any difference in most sub-adults compared to full adults;

• black markings on the old rectrices, often isolated limited black spots near the centre, although most rectrices are moulted to completely white tail-feathers by September and sub-adults with old rectrices are rare (but see image 5);

• p10 with a small mirror, p9 lacking such a mirror and very small white tips on the top of the outer primaries. By August, these tips are worn away and the primaries are brownish, strongly bleached and together with the primary coverts, the outer wing looks brownish (see this preening sub-adult, image 10). In 5cy birds, these outer primary coverts are still not really adult-like, rather blackish with white tips in some cases (see e.g. this sub-adult -presumed 5cy- in image 11);

• the grey centres of the old secondaries show a brown hue and the tips are abraded. Also, the first moulted secondaries (the outer ones, i.e. S1-S10) may, when they are still not shed, show blackish centres or fine vermiculation at the top half of the feather (see e.g. image 11, where the secondaries are just visible in the open wing);

• tertials, outer greater coverts or outer lesser coverts (in the carpal edge) show old adult-like feathers with a relative strong brown hue, obvious much stronger than expected in full adult coverts. See image 3.

Used as a single characteristic, the evidence is not very convincing, but individuals that combine these characteristics are probably 4cy or 5cy birds.

Arrested autumn primary moult in sub-adults
In autumn, LBBGs have to decide between moulting the primaries on the breeding grounds or start migrating and shed primaries after arrival in the winter quarters (or main stop-overs). This decision is made on individual considerations and strategy. Ring recoveries revealed that birds (of the same age), born in the same colony, may still be in the Netherlands by early November, while others can be found as south as Morocco. And the strategy may be changed by individuals as well; migrating south as 1cy bird, but stay in the Netherlands in 2cy. Why migration strategy differ so much between two individuals so closely related is unknown, and probably the answer can be found in their ancestors' gene flow. Whatever, the option chosen, opportunistic or conservative, it involves the primary moult schedule. By the end of August, LBBGs leave the Netherlands rapidly and at Maasvlakte numbers fall from several 10.000's to just a few 1.000's within two weeks (end of August, early September). Within those migrating groups, many adults and sub-adults leave with complete old remiges, or arrested primary moult at p1 or p2. But how to prove this hypothesis? 

It's most likely that exactly those sub-adults and adults with suspended and arrested moult are very keen to move south as quick as possible and they probably do not take long breaks on minor stop-overs. For exact numbers on suspended moult, maybe best bet is Portugal or other main stop-overs which are used for a longer time (long enough the fulfill primary moult and find enough food) and check new arriving birds. Another option is video-tape migrating birds from rocky outcrops, where they pass at short distance, and base estimates on the video-stills.
So far, detailed surveys on suspended and arrested moult in (near) adult graellsii/intermedius LBBGs in autumn is lacking, but it certainly occurs, as some pictures show (see e.g. image 20 and image 21). In other sections, arrested moult or expected suspended moult is mentioned as well.