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Identification Pitfalls and Assessment Problems*

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Identification Pitfalls and Assessment Problems* Identification pitfalls and assessment problems* This series, which started in January 1983 (Brit. Birds 76: 26-28), is not intended to cover all facets of the identification of the species concerned, but only the major sources of error likely to mislead the observer in the field or the person attempting to assess the written evidence. The species cov­ ered are mostly those which were formerly judged by the Rarities Committee*, but which are now the responsibility of county and regional recorders and records committees. 12. Surf Scoter Melanitta perspicillata he decision of the British Birds Rarities Committee to delete Surf Scoter Tfrom the list of species it considers, with effect from 1 st January 1991, re­ flects the increase in occurrence of this Nearctic duck on this side of the At­ lantic. In the 16-year period 1958-73 there were 28, but there has been an upsurge since then with at least 246 in the following 16 years, from 1974 to 1989. Surf Scoter nevertheless continues to be a rare bird in England and Wales. It has become increasingly commoner in Scotland, where small parties are now regular at favoured sites and where, for example, a minimum of 11 different individuals was recorded at one such site on the East Lothian coast in 1989. These parties have included paired birds, and there has been much display as well as aggressive interaction between paired males and unpaired males. There must be a prospect of breeding occurring sooner or later on this side of the Adantic—if it has not already occurred. With good views, there should be no real problem widi the identification of adult male Surf Scoters. They are slightly shorter in length than Velvet Scot­ ers M. Jusca. Compared with that species and with Common Scoter M. nigra, they are proportionately bigger-headed and appear shorter-necked, squatter and bulkier. The proportionately bigger head of Surf Scoter is accentuated further by its larger bill, which is obviously deeper than that of Velvet or Common, and which has a markedly swollen base. The biE, a long sloping forehead and a flattish crown combine to give Surf Scoter a head profile remi­ niscent of Eider Somateria moUissima. This head profile is quite different from that of Common Scoter, which has a smaller and shorter bill and a very rounded head. The profile of Velvet Scoter is intermediate, with sloping bill and forehead, but a more rounded crown. The multicoloured bill of Surf Scoter, with yellow, black, white and orange, is readily visible at long range, even if the detailed pattern can be discerned only at closer range. Also strik­ ingly visible at very long range are a large shield-shaped white patch covering most of the nape and a smaller white patch on the forehead. During summer and autumn eclipse, die white nape patch disappears or is obscure, but the forehead patch is still prominent. Surf Scoter, like Common Scoter, lacks the white secondaries of Velvet Scoter. When Velvet is on the water, the white *This paper, like those earlier in the series (Brit. Birds 76: 26-28, 78-80, 129-130, 203-206, 304- 305, 342-346; 77: 412-415; 78: 97-102; 81: 126-134; 84: 145-148; 85: 21-24), is a publication of the Rarities Committee, which is sponsored by CARI, £EISS—Germany. 14 \Brit. Birds 85: 437-439, August 1992J 437 438 Identification of Surf Scoter secondaries may show as a small patch or line on the wing, but they are often hidden by the scapulars. The aE-black wings of Surf are best confirmed in flight or when, as Surf Scoters frequendy do, the bird stretches up from the water and flaps its wings. First-summer male Surf Scoters have the same general appearance and structure as adult males, but the bill is less developed and not so swollen. The bill colours are duller and paler, the nape patch tends to be smaller and duller, the forehead patch is absent, the retained juvenile wing feathers are browner, and the belly is pale. When seen together in flight, die wingspan of Surf Scoter is noticeably shorter than that of Velvet Scoter. Surf Scoter also tends to sit lower in the water than do other scoters. In summary, males are straightforward to identify on bill and head struc­ ture and shape, bill colour, one or two striking white head patches and die absence of white secondaries. So what possible pitfalls could there be? The an­ swer usually lies in observers being unwary or too 'keen' when looking through distant scoter flocks in rough seas when birds are bobbing into view only momentarily. On several occasions in such circumstances, I have seen observers convince themselves, particularly when Surf Scoter is known to be present, that they have seen the striking white head patch of Surf, when they have almost certainly seen die flashing white secondaries of a Velvet Scoter wing-flapping as it bobbed into view or the white of a Long-tailed Duck Ckm- gula hyemalis. Conversely, it is also worth noting that the smaller nape patch of first-summer males may not be visible when the bird is in full profile. This could lead to such birds being overlooked in a distant scoter flock. Adult females and juveniles are more difficult. Juveniles are similar to adult females, but are paler than the dark sooty-brown females, lack the pale nape patch which is present on some females, and have pale bellies. Assessment problems arise when the bill and head shape and pattern are not well enough described. In such cases, and where die lack of white secondaries has not been noted, Velvet Scoter may not be ruled out. Where absence of white secon­ daries has been noted, for example on a bird flying past, the question is whether Common Scoter can be eliminated. Female and juvenile Surf Scoters share the same Eider-like bill and head profile of males. Aldiough die dark greyish-black bill is not obviously swollen as it is with males, it is still deeper- based and heavier than that of Velvet. At least some first-winter females have a less well-developed bill (and therefore also a less extreme head shape) dian adult females (Peter Lansdown verbally). The head pattern is variable, but all individuals show a darker crown and paler 'face', producing a capped effect which is rarely, if ever, shown by Velvet. The paler face of Surf Scoter is, however, still dark compared with the very pale and strongly contrasting face of Common Scoter. Female Surf can show a pale nape patch, which is never shown by Velvet or Common Scoters. This can be as extensive as die nape patch on males, aldiough duller, and is tiien readily visible at long range. On other females and on juveniles, it is not present and these birds require partic­ ular care to ensure that they are not overlooked as Velvet Scoters. Like Vel­ vet, the two off-white patches on the face, one in front of and one behind the eye, are variable and can be absent. At very close range, and when present, a difference can often be seen in die shape of die patch in front of die eye on Identification of Surf Scoter 439 Surf and Velvet Scoters. On Surf, the anterior edge of the patch is more or less straight and vertical, whereas the whole of the patch on Velvet is oval or egg-shaped. Some Common Scoters, especially juveniles in autumn, can show dusky patches on their pale faces, producing a pattern which could be con­ fused with that of Surf Scoter, but the smaller bill and different head profile should preclude confusion. The key to the safe identification of female and juvenile Surf Scoters is con­ firmation of the absence of white secondaries and precise observation of the bill and head profile and head pattern. Observers should also be careful not to assume that a female accompanying a male Surf Scoter must also be that species. In the Firth of Forth, unpaired male Surfs have frequently associated very closely with female Velvets, and have also joined in with displaying groups of male and female Common Scoters. Females should therefore be confirmed on positive identification features and not by association. Killian Mullarney (L\itch Birding 5: 24-25) drew attention to the diving and wing-flapping behaviour of scoters. Briefly, Surf dives with partially opened wings and often jumps forward; Velvet dives with partially opened wings, often revealing a flash of its white secondaries, but with no jump; and Com­ mon dives with closed wings and an energetic jump. When wing-flapping, both Surf and Velvet keep their heads and bills pointing above the horizontal and their necks rigid, whereas Common usually droops its neck with a down­ ward thrust during the wing-flap. I agree with these general differences, but would stress, as Killian Mullarney did, that such behavioural characteristics should be used only as good indicators of the species concerned. Positive iden­ tification should then be made, using the criteria described earlier in this sum­ mary. If that proves impossible, an observer should accept that it 'got away'. Firm claims need to be based on structure and plumage, not behaviour. ALAN BROWN 23 King's Court, Longniddty, East Lothian EH32 OQP A paper by Jon Dunn on the identification of Surf Scoter is in preparation and will be pub­ lished in a future volume of British Birds. EDS. .
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