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Kijk uit naar/voor donkere NoSto´s!

27 januari 2009  ·  15688 × bekeken

Vanaf half december 2008 zijn in het noorden van Schotland donkere Noordse Stormvogels Fulmarus glacialis gezien. In de eerste twee weken van dit jaar zijn ook in Nederland enkele donkere Noordse Stomvogels gezien en dood gevonden. Of en hoe deze potentiële ´wreck´ van Noordse Stormvogels met daartussen mogelijk exemplaren van de donkere vormen zich zal ontwikkelen wordt bijgehouden op de website van de Nederlandse Zeevogel Groep (www.zeevogelgroep.nl). Hier kan men ook een korte handleiding voor het vaststellen van de kleurfases vinden. Eventuele nadere informatie wordt hier on-line beschikbaar gesteld. Het lijkt er overigens op dat er na 13 januari geen Noordse Stormvogels meer zijn waargenomen of dood gevonden in Nederland.
Anthony McGeehan schreef in Dutch Birding 20:66-68 (1998) een short note over de zogenoemde ´Blue Fulmar´ (donkere NoSto), waarin hij niet alleen kort ingaat op de kleurfasen van Noordse Stormvogels an sich, maar ook waarschuwt voor... verwarring van ´Blue Fulmar´ met ´donsstormvogel´ Pterodroma mollis/feae/madeira (en deserta)!

Gijsbert van der Bent

 

Dutch Birding jaargang 20 nummer 2, pagina 66-68

Appearance of 'Blue Fulmar' and potential confusion with
'soft-plumaged petrels'

Depending on weather conditions, large passage movements of Fulmars Fulmarus glacialis occur around the coast of western Ireland, chiefly in early autumn. Among the many 100s of passing 'typical' Fulmars, occasional lone 'Blue Fulmars' (dark morph) also occur. Colour variation in the North Atlantic Fulmar has been described in four colour phases (Fisher 1952, van Franeker & Wattel 1982): 'double light' (LL) for birds with fully white head and underparts; 'light' (L) for birds with a greyish coloration on head/neck and sometimes on the underparts, but not on the breast; 'dark' (D) for birds which also have a dark breast; and 'double dark' (DD) for darker, uniformly grey birds. However, in practice and under field conditions, what is light and what is dark is a matter of opinion. In fact, in the most recent attempt to rationalise a colour phase system (van Franeker 1995), a functional categorisation into light (LL) and coloured (L, D and DD) was arrived at. During the last 10 years, I have been fortunate enough to see c 40 'coloured' Fulmars which spanned a range of darkness. While some were quite similar in overall shade to typical birds, they could quickly be distinguished by their almost concolorous upperparts, with the colour of the head almost matching the grey back, wings and tail. For many birds, a useful analogy is to think of a Blue Fulmar as resembling a typical Fulmar seen through smoked glass. This is most apt for darker individuals which, in life, can look almost uniformly lilac-coloured. However, if one ground rule for detecting Blue Fulmar is head colour, then another is underwing darkness.
Because all Fulmars glide close to the sea, their underwings are subject to a changing amount of shadow, but Blue Fulmar really does have variably dark, smoky-grey underwings. Even lightly-coloured Blue Fulmars show dark underwings. In fact, irrespective of how dark the bird is, the underwings appear to be its darkest part. While Fulmar has limited dark on the underside of the outer primaries and adjacent primary coverts, with a narrow darker 'strip' along the underside of the secondaries, the remainder of the underwing is pure white. Blue Fulmar has grey underwings with grey axillaries and underwing-coverts. In particular, the under primary coverts can stand out as noticeably dark, almost blackish, as can the outer primaries. In turn, this makes the inner primaries stand out as an often conspicuous pale wing flash. Depending on distance, other differences from typical Fulmar can be seen, which include head pattern and bill markings. Blue Fulmar often shows an obvious blackish 'shadow' around the eye. Typical Fulmar also possesses this mascara-like mark at the front of the eye, but Blue Fulmar not only shows a more extensive patch at the front, but also may have a dark area around and behind the eye. On several individuals that I have seen, the bill of Blue Fulmar has shown a continuous dark band which runs as a diagonal from the upper mandible to the gonydeal angle on the lower mandible. This can vaguely recall the bill pattern on an adult Ring-billed Gull Larus delawarensis. Bill colour also tends to be yellower on Blue Fulmar. I accept that there may be no merit in pointing out these bill pattern details, since this could simply be individual variation (the view of Bernard Zonfrillo).
Taking all of the above together, is it possible to confuse a Blue Fulmar with a 'soft-plumaged petrel' Pterodroma feae/madeira/mollis? I believe it is, especially as the peak time for sightings of soft-plumaged petrels, the month of August, also witnesses the largest movements of Fulmars around the Irish coast which, drawn from a wide catchment of the northern Atlantic, usually contain a few Blue Fulmars. For an observer unfamiliar with Blue Fulmar, it may come as something of a surprise to encounter a lone seabird flying in strong winds with an easy, languid style and flashing dark underwings (a 'diagnostic' feature of soft-plumaged petrels). The classic Fulmar shape can be disguised because the bird is grey and uniform and, depending on the strength of the wind and state of the sea, the bird may be gliding and arcing high, and holding its wings in a manner which makes them look quite pointed and un-Fulmar-like (as with the Blue Fulmar depicted in Dutch Birding 15: 91, plate 52, 1993). Rare seabirds also have the unfortunate habit of flying alone, making on-the-spot comparisons with commoner species impossible. A lone Blue Fulmar has caused consternation on at least one Irish seawatch (pers obs). Unfortunately, field experience of soft-plumaged petrels from land-based seawatching is very difficult to acquire. I have been extremely lucky to see three individuals, which can only stand as a basic introduction to the subject. Nevertheless, compared with even the most troublesome Blue Fulmar, there are differences. The flight style is distinctive and so too the constantly angled, boomerang-shaped wings. More explicit field points are a pure white belly and underparts and almost evenly dark, blackish underwings. Soft-plumaged petrel also has white 'armpit' triangles (which may be difficult to see) and lacks the normally striking pale flash on the inner primaries shown by Blue Fulmar. On the latter, this is visible both from below and above. The upperwings of soft-plumaged petrel not only lack any equivalent wing flash, but they are significantly darker than in any Blue Fulmar I have seen.
I am grateful to Bernard Zonfrillo for supplying me with references and for helpful discussions.

References
Fisher, J 1952. The Fulmar. London.
van Franeker, J A 1995. Kleurfasen van de Noordse Stormvogel Fulmarus glacialis in de Noordatlantische Oceaan. Sula 9: 93-105.
van Franeker, J A & Wattel, J 1982. Geographical variation of the Fulmar Fulmarus glacialis in the North Atlantic. Ardea 70: 31-44.

Anthony McGeehan, 75 Lyndhurst Avenue, Bangor BT19 1AY, Northern Ireland

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