Kleurfasen van de Noordse Stormvogel Fulmarus glacialis in de Noordatlantische Oceaan
Sula , Volume 9 - Issue 3 p. 93- 106
Colour variation in the North Atlantic Fulmar has been described in four colourphases (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 in which also the breast is greyish; and "double dark" (DD) for darker, uniformly grey birds. This colourphase system has frustrated quite a few fieldobservers because subtle tinges of grey are not easily seen on distant birds dashing between waves or on dirty decaying corpses on beaches. Two Atlantic subspecies have been described (Salomonsen 1965): F.g. glacialis for mainly ’dark’ high arctic populations and F.g. auduboni for ’light’ more southern populations. The description in ’light' and ’dark’ has been somewhat confusing (see box terminology on page 94), as the actual difference between populations is that they are either over 99% LL or over 92% L, D, and DD (most of which L or D; figure 1). The only exception to this pattern is a small population near Scoresbysund in east Greenland. Thus, in studies outside the breeding colonies there is little additional value in using the four colourphase system. It is proposed to use a functional categorization into light’ (LL) or ’coloured ’ (L, D, and DD) and characteristics are described (figures 2-3; Table 1 which is repeated/translated below). A grey underwing, most pronounced on axillaries and smaller arm coverts (and connected to a grey flank and side of rump) is a good character to separate all coloured birds from light ones which have a white underwing. In flight, a gradual change in colour between the hind-neck and the grey mantle identifies a coloured bird, even if the head itself appears quite light. Usually, the nape and hind-neck are clearly grey in coloured birds in flight. LL-phase birds have an abrupt border between the white neck and grey mantle. In birds sitting on water or land, the colour of head and neck may be less obvious. Even D-phase birds may have a very light appearance: in such cases it may be helpful to look at the side of rump and flank-feathers protruding over the folded wing: in lightly coloured birds these are usually clearly darker grey than the remainder of the body (in light phase birds they are bright white, like other body-feathers). There are some other characters in colour of bill, dark eye-patch, and shades of grey on upperparts that could assist in catching the observer’s attention, but these cannot be used as decisive field characters. Colour variation in the single Pacific subspecies (F.g. rodgersii) is much stronger than in the Atlantic, with dark birds attaining a Sooty Shearwater-like colour. Opposite to the Atlantic, populations with light phase birds are found north of those with mainly coloured birds. It is not fully certain that distinctive characters between 'light' and 'coloured' birds in the Atlantic are directly applicable to Pacific birds: however, the majority of Pacific Fulmars belongs to the light or very dark extremes, with relatively few of the intermediate shades that complicate colour identification in the Atlantic Fulmar.
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