The Isabelline Wheatear Oenanthe

Transcription

1 Bradshaw: Moustached Warblers in Britain From the Rarities Committee s files Bradshaw: Moustached Warblers in Britain Identification of autumn Isabelline Wheatears The Isabelline Wheatear Oenanthe isabellina is surely one of the most difficult of British rarities to identify. This is due partly to similarities with Northern Wheatear O. oenanthe in autumn plumage, partly to the variability of Isabelline and Northern Wheatears, and partly to varying and sometimes contradictory statements made in the identification literature. It is, however, not only the identification of Isabelline Wheatear that is difficult, but also the assessment of claimed records, which can be an extraordinarily problematical task, sometimes made more so by the receipt of inadequate material on which to pass judgment. For species that combine great scarcity with the degree of subtlety in characters exhibited by Isabelline Wheatear, we demand the most detailed notes, together with, where possible, identifiable photographs of the individual concerned. The major identification features of Isabelline Wheatear were discussed by Clement (1987), and these are being updated for the text of his forthcoming book on the identification of wheatears. Corso (1997) took a novel approach to this identification problem by investigating the variability of Isabelline Wheatear. While we would not necessarily agree with all of its findings, that paper does provide an essential reference for anyone interested in this identification challenge. The BBRC files have details of 16 accepted records, as well as several reports that were rejected. I examined descriptions and photographs of the autumn records in an attempt to identify common themes occurring in the accepted descriptions, and then assessed whether or not these points of detail were present in the records that had been found unacceptable. Perceived wisdom has it that the identification of Isabelline Wheatear in autumn can be made by using the following criteria, all in comparison with Northern Wheatear (P. Clement in litt.): 1. Structural Larger size Longer legs Shorter tail Stouter bill Shorter primary projection Slightly larger head with flatter crown 2. Behavioural More upright stance Bounding or running gait Head-bobbing and tail-wagging 3. Plumage Lack of contrast between the upperparts and wings Lack of contrast between the upperparts and underparts Supercilium whiter and more prominent in front of eye, fading quickly behind Ear-coverts almost concolorous with rest of upperparts Lores variable, depending on angle and wear, but eye-stripe fading behind eye Lack of contrast in wings, especially tertials and coverts White of rump starting lower down the back Broader terminal black tail band and shorter stem of T Whitish underwings Isolated black alula Using Corso s analysis, the features showing least variability and highest reliability, in no particular order of ranking, are: (a) shorter primary projection; (b) supercilium not flaring behind eye; (c) ear-coverts of same colour as rest of upperparts; (d) general lack of contrasts in pattern of the whole bird; (e) lack of contrast in upperwing; (f) isolated black alula; (g) white underwing; and (h) tail pattern. Corso (1997) felt that the variation in size and shape within Northern and Isabelline Wheatears, coupled with the necessity for direct comparison and 488 British Birds 93: , October 2000

2 significant experience of both species, meant that almost all structural and particularly behavioural features, such as tail-wagging and gait, were of little value in identification of an individual bird. In Britain, this problem is compounded by the fact that structural and size variations of Northern Wheatear are even greater than in Italy, as Britain is regularly visited by moderate numbers of the larger Greenland Wheatear O. o. leucorhoa; this subspecies is not only larger, longerlegged and larger-billed than the nominate race, but is also often paler and frequently tends to show rather more black on the tail. Moreover, observers in Italy see both Isabelline and Northern Wheatears with some regularity, and often have the opportunity of viewing them side by side. In Britain, by contrast, many wheatears are seen by birders for whom Isabelline is not an everyday occurrence, and a putative Isabelline may turn up without an accompanying Northern Wheatear for direct comparison. Descriptions of most of the accepted British records of Isabelline Wheatear include all of the features highlighted by Corso (1997), and many of the others. British observers set a lot of store by structural comparisons, with most descriptions detailing a large-headed, robust, pear-shaped wheatear with upright stance, long legs and short tail, giving a pot-bellied look. The majority of descriptions include mention of the isolated black alula, the white underwing (although admitting how difficult this was to see), and the broad black tail band and restricted white on the rump. Most observers seem unaware that this last feature is due to the fact that the sandy-brown colour of the mantle extends onto the upper rump of Isabelline Wheatear, creating a more restricted area of white than on Northern Wheatear, and they believe that this is a result of lack of white on the tail (plate 288). Interestingly, all the records that have been deemed unacceptable also detail all these structural and plumage features. What, then, are the criteria that separate the acceptable from the unacceptable? In the series of records that I examined, the most important factor was the level of critical detail provided on the patterns of the wings and the head. Autumn birds accepted 288. Isabelline Wheatear Oenanthe isabellina, Kilnsea, East Yorkshire, October as Isabelline Wheatear not only had broad pale buff fringes, lacking any rusty tones, to the tertials, secondaries and coverts, but the ground colour of the feathers was also not black or dark brown but mid brown or greybrown, thereby lessening contrast with the pale fringes.this produced a less contrasting, more uniform wing. When the wing was closed, the darker centres to the lesser and greater coverts and secondaries were usually completely obscured by the fringes. The centres of the median coverts, however, sometimes showed through as indistinct dark freckles near the bend of the wing, but this was quite different from the string-ofpearls effect often seen with the darkcentred median coverts of Northern Wheatear. The mid-brown centres of the tertials are always visible, but lack the obvious contrast between the blackish centres and pale edges shown by Northern Wheatear.The general effect of the browner centres to the wing feathers, combined with the broad buff fringes, is to reduce the contrast between the wing and the rest of the upperparts, producing a more uniform appearance. Descriptions of the head pattern varied, but almost all mentioned a supercilium British Birds 93: , October

3 which was most prominent before the eye, rapidly fading behind. This was accentuated by a dark line (varying from thick black to indistinct brown, flecked pale) across the lores, which disappeared, or became only a shadow, along the upper border of the earcoverts, which were the same colour as, or only a shade darker than, the crown and nape. Most of the acceptable records also detailed how the facial pattern changed considerably when viewed at different angles and in different lights. In particular, the dark loral line appears more prominent when the bird is viewed front-on, as the observer is looking down the forward-pointing darkbased feathers. Often, the birds were thought to be extremely plain-faced until seen headon, when a noticeable supercilium became apparent. Four notable exceptions to these rules were found, all involving undoubted Isabelline Wheatears. The first, and perhaps the most dramatic, was the small individual at Kilnsea, East Yorkshire, in October 1990 (Brit. Birds 84: 483). This bird (plate 289) was the same size and shape as nearby Northern Wheatears. It was neither longlegged nor particularly short-tailed, and the bill was quite weak, although photographs suggest that it could also give the opposite impression (plate 290). Because of its size and colour, and an almost complete black tail, it was initially misidentified as a Desert Wheatear O. deserti before John McLoughlin correctly identified it, on the basis of the general plumage, head pattern and pale underwing. Plate 289 shows the uniform plumage, the relatively plain head pattern, with an indistinct supercilium before the eye only, and a dark loral line that fails to extend behind the eye.a similar bird was present on Whalsay, Shetland, in September 1994 (Brit. Birds 88: 534). Once again, this individual was not noticeably larger or longer-legged than Northern, nor was the stance particularly different. It, too, was identified predominantly on plumage characteristics. The third exception to the rules involved a well-watched bird on Gugh and St Agnes, Isles of Scilly, in October 1991 (Brit. Birds 85: 537).This individual was, in many ways, a typical Isabelline in size and structure, and the head pattern was extremely variable, depending on light and also on the angle at which it was being watched. Plate 292 shows a characteristic view of it, with the supercilium more marked in front of the eye and narrowing quickly behind, uniform earcoverts and an almost negligible eye-stripe behind the eye; also shown is the typical primary extension, the mid-brown centres to the tertials, the centres to the secondaries and greater coverts completely obscured by the broad pale fringes, and the darker freckles around the median coverts. Plate 291 of the same individual, in different light, 289. Isabelline Wheatear Oenanthe isabellina, Kilnsea, East Yorkshire, October 1990 (same individual as in plate 288). 490 British Birds 93: , October 2000

4 however, shows a supercilium at least as prominent behind the eye and, if not flaring, at least remaining parallel for the length of the ear-coverts. This photograph also shows the extent of the white on the rump, the upper border falling between the tips of the middle and longest tertials, whereas, on Northern Wheatear, the corresponding mark is higher, usually just above the tip of the middle tertial.the final unusual record was of a bird trapped at Girdleness, Grampian, in October 1979 (Brit. Birds 73: ), which displayed a supercilium that was whiter and more obvious behind the eye than in front. What would an ideal record of an Isabelline Wheatear look like? Well, first of all, the bird should have been seen by several competent observers, and the record should preferably be supported by photographs. This is not because we disbelieve single observers, but, for subtle identifications such as this, it is possible for a lone observer to go off at a tangent and become convinced that he or she is watching an Isabelline Wheatear when it is actually a large, pale Northern Wheatear. This is slightly less likely to occur when several people see the bird. Groups of observers should always expect us to want to compare several separate descriptions when assessing a record, looking for different emphases that build up the full picture. Photographs also allow some independent scrutiny.the descriptions should be detailed and should concentrate at least as much on plumage as on size and jizz. It is not sufficient merely to say that there was little contrast on the wing, as we need the reasons why this contrast was absent. Similarly, we do not expect to read simply that there was an obvious black alula : we need to know how this related to the other parts of the wing, what was seen on the primary coverts, why it stood out so much, how many feathers were involved, and how often the alula was seen. We do not require just a simple description of the head pattern, but need to know how it varied in different views. Alongside this, we also need details of size and shape. An example of an ideal Isabelline Wheatear description was published by us in 1993 (Dean 1993). It is important to remember that there are two accepted British records of individuals that failed to show the structural characteristics of typical Isabelline Wheatears, but none involving birds of which the plumage was imprecisely described. References Clement, P Field identification of West Palearctic wheatears. Brit. Birds 80: , Corso,A.1997.Variability of identification characters of Isabelline Wheatear. Dutch Birding 19: Dean, A. R From the Rarities Committee s files: Isabelline Wheatear in Scilly. Brit. Birds 86: Isabelline Wheatear Oenanthe isabellina, Kilnsea, East Yorkshire, October 1990 (same individual as in plate 288). British Birds 93: , October

5 291 & 292. Isabelline Wheatear Oenanthe isabellina, Gugh, Scilly, October Tynemouth Place, Tynemouth, Tyne & Wear NE30 4BJ The British Birds Rarities Committee is sponsored by Carl Zeiss Ltd. 492 British Birds 93: , October 2000