The article of Lars Jonsson in BW 11-8 (1998) gives excellent insight in the features and moult strategy of Baltic fuscus. Without doubt, it's still the best article written about nominate fuscus and it's ID and many bird-watcher look closer at local gull flocks since. In his article, Jonsson mentions some features and plumages which (when combined) should highly advocate fuscus over intermedius and graellsii. However, it remains to be seen whether the points he mentions fully exclude intermedius. Except for upper-part colouration and jizz, arrested moult is such a feature worth looking for. As stated in his article and in the Discusfus Section at this site, arrested moult may be a common feature in nominate LBBG fuscus from Sweden and Finland, particularly in immature birds.

However, this phenomenon may be overlooked in most graellsii and intermedius LBBG. First field research in the Netherlands estimates arrested moult to occur in about 4% in (predominantly immature) graellsii and particularly intermedius LBBG. This figure is based on a very rough count of "only obvious" suspended moult in resting LBBG in 2001. Nevertheless, the existence of arrested moult on such a scale makes it more difficult to fully exclude intermedius once one come across an immature dark, gentle bird with arrested moult in a resting group of LBBG in Western Europe in May or June. One should always consider the possibility of intermedius combining the features of arrested moult, gentle, elongated structure and dark fresh coverts and scapulars.

But what about dark adults in autumn? In the Discusfus Section, it's mentioned that quite some adult LBBG from Western Europe may leave the colony without moulting a single primary. They originate from local Dutch colonies in the Netherlands and we believe some of them to be intermedius (not fuscus); see the tables in the sections Adults in August and Adults in September). Old primaries may still be present in the first half of September, but is nevertheless very unusual. This will give a late completion date of primary moult into late March. To find support for this thesis is very difficult and it is hard to prove, simply by the fact you won't find these adults with suspended moult anymore in NW Europe; they move on quickly. Research should be done at the first stop-over sites along the Atlantic coast or further south. 

Arrested moult in 3cy LBBG graellsii. Picture Peter Stewart, 2004. Grey- tone far too pale for typical fuscus. At the winter quarters, the inner primaries (P1-P6) and all secondaries were replaced for third generation feathers. This spring, the complete moult started and was initiated again at P1, now at P5. The worn third generation P6 is still present, as are the very worn and abraded P7-P10.

But one thing is certain: the primary moult strategy for adult Dutch intergrades LBBG in the Netherlands, graellsii LBBG from Britain and probably intermedius LBBG from W Scandinavia is highly variable. This can be seen in returning birds in spring and breeding birds in the colony. On this page, we like to show photographs of graellsii LBBG with suspended primary moult in the inner-hand in summer. 
Extensive research has been done in LBBG colonies in Britain by Peter Stewart; research covering several yeas now and resulting in numerous in-the-hand images of LBBG wings. This research shows that the Dutch research (4% arrested moult in immatures) under-estimates the occurrence of arrested moult in Western LBBG. 

Peter Stewart comments: "on your page showing a probable fuscus in NW France May 2001. 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."

In the near future we are happy to show more footage and research results and we try to find a way to extend this research to other places and LBBG colonies. Peter Stewart comments on the tables presented in the Adults in September: the latest dates for 10 old primaries in my study is in the first half of August and involves two birds only. My samples involve several hundreds of birds in the hand. All old primaries present in the first half of September, surely is very unusual.
Of course, we are very glad to be able to show a few of the images, all taken in Britain, by Peter Stewart. Many thanks to him for sending the research results and footage. There will sure be some identification and aging errors at this page, for which I accept full responsibility. Please let me know if you found one and all comments are welcome.

Here, a photograph of a graellsii with suspended P1-P3. This was taken on the 18 June. Peter Stewart has eight birds on his database showing this pattern between the 16 June and 8 July. Seven of the birds are adult and the eighth bird is a 3^rd summer. 
Peter suspects the odd bird will suspend up to p4 as he has birds that have renewed p1-p4 and had recently dropped p5. Many other birds will appear to start moulting normally after suspending p2. Click the thumbnail (and all following thumbnails) for Peter's comments.

Here is a bird with suspended moult in p1&p2.

Here is one of the odd moulting birds Peter came across. This bird was brought in sick to a local wildlife sanctuary in November 2001. This bird had suspended p1 to p5, and in both wings p5 shows obvious fault bars.
Unfortunately this bird soon died, so we will never know if it would have eventually dropped p6 and carried on moulting normally. Peter says: "I have observed all sorts of unusual primary moult patterns in birds that have obviously undergone stress or sickness during the moulting season. Partially grown feathers are also obvious in sick birds as my next image will show."

Here is a bird with suspended p1. This would not be obvious the field and easily overlooked in the hand, especially if the other primaries show little wear. We have undoubtedly overlooked birds with suspended p1 in April in the past, when this image was taken. Peter says: "We now look closely at all birds at all times of the year."

This image was taken in January 2001. This bird had obviously undergone some sort of stress during the breeding season. Peter: "We caught a large number of graellsii during January 2001, many showing fault baring in the primaries, but not so extreme as in this sample. Either there was a shortage of food near breeding grounds, or possibly sickness (botulism) may have been responsible. There was much botulism around in the previous year, and some birds do survive the effects. But most perish."

Undoubtedly, some overlap exists between the primary moult in fuscus and intermedius. Peter catches many LBBG in Britain and comments on his picture: "This photograph is of a very dark individual we trapped on the 17 August 2000. The bird has just dropped P1-P3, thus if seen in the field would undoubtedly been claimed as nominate fuscus. To me this bird had recently arrived on passage and commenced its primary moult. We have only one record of an adult graellsii exhibiting this same score and pattern in August. Usually it is only immature birds which exhibit this pattern and only during late May - June.
I have also made the following comment in my paper "According to Baker (1993) "Some (Fenno-Scandia) birds are known to arrest wing moult so that pre-breeding moult includes completion of primary moult, the inner ones being replaced twice during the one moulting cycle." However, there is no evidence, as yet, to suggest that this behaviour applies to British graellsii."

It would be nice to hear from Scandinavian birders the evidence for the above phenomenon. It would also be nice to know whether the evidence for this statement was based on trapped and ringed individuals.

Peter continues: "Suspended moult mainly involves adult birds and during the breeding season. It is not common in immature birds. We have found resumption of moult after suspension in only three second summer birds, and both suspended and resumption of primary moult in seven third summer birds. No first summer birds have been found in suspended moult. Suspended moult therefore appears to be mainly associated with breeding birds. 
Though having said that, I have found captive non-breeding adult birds in suspended moult. About 10% of these adults caught during the breeding season were found to be moulting normally, thought whether they were breeding or not was not established."

Peter comments: "The photograph shows how our 2^nd-summer bird would appear prior to primary moult. This photograph was taken in January when our BTO age code changes from 5 to 7, progressing from 2^nd-winter to 2^nd-summer."

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