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Identification Pitfalls and Assessment Problems

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Identification Pitfalls and Assessment Problems Identification pitfalls and assessment problems 6 Aquatic Warbler Acrocephalus paludicola Records of this species can be as difficult to judge as the species itself can be to identify: the main problem being that bright young (juvenile or first- winter) Sedge Warblers A. schoenobaenus superficially resemble Aquatics and are a major pitfall for the unwary. 145. Above, first-winter Aquatic Warbler Acrocephalus paludicola (note 'tiger-stripe' upperparts, two pale 'tramlines', streaked uppertail-coverts, and pointed tail feathers), Lincolnshire, September 1971 (R. B. Wilkinson) 144. Left, adult Sedge Warbler Acrocephalus schoenobaenus (note relatively unmarked back and rounded tail feathers), Kent, June 1977 342 [Brit. Birds 76: 342-346, August 1983] Identification pitfalls: Aquatic Warbler 343 146.Juvenile Sedge Warbler Acrocephalus schoenobaenus, Netherlands, July 1970 (P. Munsterman) 147. Adult male Sedge Warbler Acrocephalus schoenobaenus (note unmarked underparts and, on this individual, a 'pitfall' crown-stripe), Suffolk, April 1982 (R.J. Chandler) 344 Identification pitfalls: Aquatic Warbler 148. First-winter Aquatic Warbler Acrocephalus paludicola (note trace of fine streaking on breast and flanks), Cornwall, August 1977 (A. T. Flumm) 149. Juvenile Sedge Warbler Acrocephalus schoenobaenus (note spotted breast), Hertfordshire, July 1980 (A. P.Johnson) Identification pitfalls: Aquatic Warbler 345 The two species are the same size, and they occur in similar habitats on migration. Whilst they have plumage similarities, there are several differences, most of which need to be seen and checked before identification as Aquatic can be regarded as having been clinched. These are: 1. Yellowish-buff upperparts (dullest on adults and yellowest, even orangy-yellow, on young birds) with long, black, almost continuous streaks on back creating a 'tiger stripe' pattern. Down the edge of the mantle, on either side of the central black stripes, are two broad, pale 'tramlines'. (Even the brightest Sedge is only buffish above, or more usually grey- brown, with short, diffuse, brownish streaks, and does not show pale 'tramlines'.) Upperparts described simply as 'pale, streaked dark brown' would be insufficient for a claim as Aquatic. 2. Bold head pattern of buff-yellow crown-stripe and supercilium, separated by clear-cut blackish lateral crown-stripes which broaden on nape. (Sedge has a brown crown, lightly streaked darker; though young birds especially can show a paler crown-stripe, this is never clear-cut and is always dark-streaked.) 3. Streaked rump and, in particular, uppertail-coverts. The streaked rump can be difficult to observe on Aquatic, but the uppertail-coverts are particularly well marked, having black centres to the feathers, with broad buff to orangy-buff edges. (There is no streaking on rump and barely any on uppertail-coverts of Sedge.) 4. Bright flesh or pearly pink legs. (Light brown on Sedge.) 5. Pointed tail feathers which sometimes produce a spiky effect to a graduated tail-end. (Individual feathers are not pointed on Sedge, and the tail is only slightly rounded, not so graduated.) 150. Adult female Aquatic Warbler Acrocephaluspaludicola on nest (note head pattern and fine streaking on sides of breast), German Democratic Republic, June 1974 (Heinz Wawrzyniak & 346 Identification pitfalls: Aquatic Warbler 6. Fine streaks on upper breast and flanks are usually present on adults, but these are less obvious on first-winters and absent on juveniles. (The reverse is basically true of Sedge: adults show no breast markings, but juveniles and to a lesser extent first-winters do, although these are more spot-like than the fine streaking of adult Aquatic.) Many of these distinguishing features are shown in plates 144-150. Over 90% of Aquatic Warblers in Britain and Ireland are in August and September, so descriptions of any claimed outside those two months should be examined very critically. RICHARD PORTER RSPB, The Lodge, Sandy, Bedfordshire SG192DL .
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