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SEPARATING RADDE’S WARBLER Phylloscopus schwarzi
AND DUSKY WARBLER P. fuscatus
RADDE’S AND DUSKY WARBLERS ARE notoriously difficult birds to observe in the field, with both species being extremely retiring and disappearing for long periods of time. This behaviour, along with the difficulty of identifying these species, means that they are two of the most frequently misidentified species on the British list. Although both occur in varying numbers each autumn, they haven’t yet shown the recent increase exhibited by Pallas’s and Yellow-browed Warblers, despite all four species breeding in the same area of Siberia. The identification of Radde’s and Dusky has always been difficult with continually changing views on what exactly is a ‘valid’ identification feature. Even a number of trapped birds have proven problematical, with wing formulas and basic biometrics between the two being remarkably similar. Apparently the width and depth of the bill are the only reliable means of separation in the hand. Ageing is virtually impossible, even in trapped birds, so it is unknown how many of the birds are immature or adults. Generally speaking, they are more similar on their breeding grounds than they are in Britain. Dusky usually appears from late September to mid-November, with a peak in late October. Over-wintering is not unknown and two were found around the beginning of 1995, which contributed to the first ever winter records. The first was at Kenwith NR, Bideford, Devon, and the second was at Bude Marshes, Cornwall. The Kenwith individual was present from December 28, 1994, to April 30, 1995. The Bude bird was present from January 11 to May 4, 1995. Radde’s occurs from late September to early November, with the middle week of October being a favoured time, although exceptions do occur as with any species.
The following concentrates on the identification of Radde’s and Dusky, with an emphasis on the plumage that is most frequently recorded in this country.
Radde’s and Dusky Warblers are both essentially medium-sized, brownish warblers with short wings, longish tails and obvious supercillia. Both can be notoriously skulking so during the normal brief views afforded observers should concentrate on the general jizz of the bird, the supercillium colour, bill size and general coloration. Radde’s Warbler is the more ground-dwelling of the two species and according to recent literature it always avoids the canopy of the trees. Although this is probably true, the Lowestoft individual of 1996 spent a short while in the very top of a tall clump of sycamores. However, the presence of a large crowd of observers may have contributed to this behaviour. Generally, Radde’s is the more secretive of the two species, frequently spending long periods deep in cover, although if left undisturbed it will happily feed out in the lower branches of an adjoining tree. Dusky Warbler is the more arboreal of the two, but can still be frustratingly elusive for long periods. Radde’s Warbler is the slightly larger and stockier of the two, with a large-headed appearance and a longer tail. This species can frequently appear bull-necked, although I have also seen them appear quite long necked on occasions. It appears a robust species with slow, predictable movements, at times recalling an Hippolais or Sylvia warbler. It occasionally flicks its tail and wings, but does so less obviously than Dusky. Dusky Warbler also occasionally hovers, a feature which is unknown in Radde’s. Dusky Warblers are slightly smaller than Radde’s, with a flatter head and a shorter tail, generally recalling a Willow Warbler in shape and size.
Radde’s Warbler has the broader-based, stouter bill of the two, with a noticeable blunt tip, at times recalling a Garden Warbler. Apparently, 1st year birds have weaker bills than adults, prompting confusion with Dusky Warbler. Dusky Warbler has a finer, more pointed bill, which is similar in shape and size to Willow Warbler. Both species have pale-based lower mandibles, with noticeably darker tips. On Radde’s the lower edge of the upper mandible is often pale, unlike Dusky Warbler which is usually all dark.
Radde’s Warblers legs are usually dull orange and heavy, with obvious joints and feet, compared to Dusky Warbler’s which vary between dark pink and mid-brown, and are unremarkable in size. The weigoldi sub-species of Dusky Warbler has all dark legs although is unlikely to reach Britain.
Radde’s Warbler has the longest supercilium of the two species, reaching from the bill and extending back onto the nape. It is broadest, although most diffuse, before the eye, and often kinked at its rear, where it ends in a point -- although the latter feature is most noticeable when the bird appears bull-necked. On Dusky Warbler the supercillium is narrow and more defined in front of the eye and square-ended at the rear. The supercilium colour difference is always a widely quoted distinguishing feature between the two species, although after my experience with the 1996 Holkham Dusky Warbler I would be slightly cautious about this feature. All the books state that if Dusky Warbler shows any contrast at all in their supercillia, they are always whiter before the eye, the reverse being the case in Radde’s. Although on the Holkham, Norfolk Dusky Warbler that I watched at close range and for 45 minutes, the supercillium was slightly but definitely whiter behind the eye. It would appear that this feature isn’t as fail safe as recently thought. So generally Dusky Warbler’s supercilium is cream / buff, totally lacking in any yellow coloration and slightly whiter before the eye (although, as I have found out, there are exceptions). On Radde’s Warbler the supercilium colour differences are most easily judged when the bird is in shade, as full sunlight tends to work out the colour. Radde’s Warbler always shows a dark grey coronal bar above the supercillium, a feature that is only rarely shown by Dusky Warbler. On Radde’s Warbler the dark eyestripe is usually darker than the bird’s upperparts, whereas it is usually the same colour on Dusky. Also, the eyestripe is poorly marked and smudgy before the eye on Radde’s Warbler, whereas it is even and consistent on Dusky Warbler. Radde’s Warbler has obvious mottled olive ear-coverts, being fainter and browner on Dusky. Dusky Warbler often shows broken white eye crescents, with Radde’s being more obscure.
Radde’s Warbler’s upperparts vary between dark green and olive. However, they can appear surprisingly dark brown, above when in deep shade, which can be a trap for the unwary. An obvious olive and yellow bird is always a Radde’s Warbler and probably a 1st year bird, occasional brown and buff birds are more problematical and are probably adult birds, however this has not been proven. Fortunately, most Radde’s Warblers are noticeably olive above and with green primary fringes, which makes them quite different to Dusky Warblers, which are always mid to dark brown above. No olive is ever apparent in the upperparts of a Dusky Warbler, even when in the hand.
Radde’s Warblers vary from dirty white to sulphur-yellow below, and always with a contrasting rufous / rusty vent. This is different from Dusky Warbler, which is more of a neutral grey below, and normally with no vent contrast. Radde’s Warblers occasionally-show a faint band of yellow streaking across the upper breast which is absent in Dusky. Both species have slightly buffy flanks, and show a pale throat, although only Radde’s Warbler has any yellow tones.
Radde’s Warbler has a more tongue-chacking call, ‘chek chek’, lower in pitch than Dusky Warbler. The call is often twice repeated which is a diagnostic feature of this species. Dusky Warbler often utters a slightly weaker version of Lesser Whitethroat‘s ‘tak’.
Radde's Warbler, Lowestoft - October 2003
Dusky Warbler, Kessingland - January 2005
Madge, S. Field identification of Radde’s and Dusky Warbler Brit. Birds, vol. 80 No 12 (1987).
Madge, S. Separating Radde’s and Dusky Warbler. Birding World vol. 3 No 8 (1990).
Bradshaw, C. Separating Radde’s and Dusky Warbler. Brit. Birds, vol. 87 No 9 (1994)
Parmenter, T., & Byers, C. A guide to the Warblers of the Western Paleartic (1991)
Lewington, I., Alstrom, & P., Colston, P. A field guide to the rare birds of Britain and Europe (1991)
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