Chestnut-sided Warbler: new to the Western Palearctic

t about 13.30 GMT on 20th September 1985, having seen a Pied Fly­ A catcher Ficedula hypoleuca at the end of the Burn of Feal on Fetlar, Shet­ land, I decided to walk up the burn, a walk I often did to look for migrants. The burn runs at the bottom of a valley which is distinctly steep-sided, giving a lot of shelter from the wind and, at the Wick of Houbie end, a fair amount of cover: docks Rumex, thistles Cirsium, yellow iris Iris pseudacorus, meadowsweet Filipendula ulmaria, and so on. The valley there is marshy and approximately 40 m wide, the sides being some 2 m high. About 200 m further up the burn, it narrows to about 20 m across, with 6-m sides. At this point there is a fence across the valley which in effect marks a change in vegetation, to short, well- grazed pasture. I found the Pied Flycatcher again at the end of the burn and then walked into a patch of irises and flushed a Garden Warbler Sylvia borin, which flew farther up the burn to a large patch of thistles. As I approached the point where I had last seen the Garden Warbler, I saw another warbler moving through the thisdes. It then flew from the thistles to the fence. It was unlike any warbler I had ever seen, having double yellow-white wing-bars, a bright green-yellow head, back and rump, and white-grey underparts. It perched on the fence with its tail erect and wings slighdy drooped. When the had worked its way to the end of the thisdes where the fence crosses the burn it flew back to the iris patch some 200 m away. Throughout the period of watching, the warbler would work its way up the burn to the point where the vegetation changed and then fly back to the Hou­ bie end, sometimes in one continuous flight, or in stages. When the bird flew the longer distances, it had a slightly undulating flight, the wing-bars were very distinct and some white was visible on the outer tail feathers. The warbler seemed to prefer to perch on vantage points, such as the fence, tops of docks, and thisdes. It would then fly down into the vegetation to find insects before returning to a vantage point. When on the fence, die war­ bler would hop short distances with its tail erect and wings drooping slighdy, flicking its wings intermittently. When its tail was held in this position, white on the undertail-coverts was very distinct. The warbler would also fly up into the air, flycatching, and dien return to near the original take-off point, very like a flycatcher.

[Brit. Bird* 86: 57-61, February 1993] 57 58 Chestnut-sided Warbler: new to the Western Palearctie After viewing the bird for about 30 minutes, I returned to the house a few hundred metres away to get my telescope, with which I was able to observe a lot more detail, including a very distinct white eye-ring and the fact that the grey-white of the 'check' extended slightly above the eye. After further observations, I returned again to the house, at about 15.00 hours, to ask my wife, Val, to come and look at the bird, as she was the only other birdwatcher on the island at the time. We both got excellent views of the bird in good sunlight. At 16.30 hours, we returned to the house and I telephoned, among others, Mike Walker, the RSPB warden for Fetlar, who was on holiday—on Fair Isle! He managed to get off and return to Fedar that evening, but arrived after dark. We went out at first light (05.30 hours) on 21st, to be joined later by other birders from the Shetland Mainland, but, despite thorough searching, the bird was not seen again. My last sighting had been at about 17.30 hours the previous evening. The weather during the period of observation had been bright and sunny, with approximately 2 oktas cloud cover, with a force 4 northwesterly wind. Although I was reasonably certain that the bird was an American warbler of some kind, I lacked a North American field guide, and it was not until my telephone conversation with Fair Isle that the species, Chestnut-sided Warbler Dendroka pensybanwa, was determined, the first to be recorded in Britain and the Western Palearctic.

Description The following description is based on' my field notes taken at the time, supple­ mented by my wife's notes. SKI; Slightly smaller than Pied Flycatcher that TAIL Dark grey, possibly with some green. it perched next to. Some white on outer tail feathers, thought at me time to be probably on outer webs, but ac­ HEAD AND NECK Forehead, crown and upper tually occurs only on inner webs (Dr A. G. part of nape bright green-yellow (moss-green). Knox in Hu.). Ear-coverts and 'cheek' grey-white, extending to just above eye and also to side of neck and UNDERPARTS Chin white, throat and upper onto lower nape. Fairly distinct white eye-ring, breast grey/white, becoming white on lower no eye-stripe. breast and belly, but greyer towards flanks. Undertail-coverts white and very conspicuous UPPERPARTS Mantle, back and rump all bright when tail held erect. green/yellow, slightly duller than head and BARE PARIS Legs grey. Bill brown and typical nape, and unstreaked. warbler shape, upper mandible appeared WINGS Primaries and primary coverts dark lighter than lower, but this probably effect of grey with possible greenish tinge. Secondaries light. Eye large-looking and dark. grey-green, edged dull yellow. Tertials greener BEHAVIOUR Percheci frequently on fence or than secondaries and with more yellow edging. other vantage point, flying down into sur­ Lesser coverts green, greater and median rounding vegetation to catch insects, also up in coverts green with two yellow-white wing-bars, air like flycatcher. Often hopped short dis­ the lower bar being slightly larger than the tances with tail erect and wings drooped, upper (boil) very distinct on open and closed sometimes (licking wings. Appearance recalling wing). Wren Troglodytes troglodytes when tail held erect. The plumage was very fresh-looking, with little wear. After consulting North American bird guides, the age was confirmed as first-year. Apart from the conspicuous chestnut flanks which give the bird its name, both adult male and female also have a boldly streaked mantle.

'The inclusion of plates 14-17 in colour has been subsidised by a donation from Carl ^eiss—Germany. 14-17. Chestnut-sided Warblers Dendroica pensylvanica, . Above, male, May 1986 (Martin S. Gamer); right and below, juveniles, August 1991 and August 1988 (Colin Brads/iaw); bottom, September 1974 (A. D. Brewer) 60 Chestnut-sided Warbler: new to the Western. Palearctk Distribution The Chestnut-sided Warbler breeds from central-eastern Alberta cast to Nova Scotia and south to eastern Nebraska, southern Wisconsin, northern Ohio, central New Jersey and in the Appalachians south to northern Georgia and northwestern South Carolina. It winters from southern Nicaragua to Panama.

Weather and other transatlantic vagrants The weather pattern during the preceding weeks had seen an increase in pres­ sure to the south, moving the track of the Atlantic depressions farther north after 7 th September. After 18th, the centre of pressure moved to the east over and southerly or southeasterly winds became more frequent towards the end of the month. No other North American vagrants had appeared in Britain in autumn 1985 before the sighting of the Chestnut-sided Warbler, but both Northern Parula Paruh ammcana and Blackpoll Warbler Dendroka striata were found on 30th September, to be followed by an October that was confidently described as: 'the best-ever single month for American landbirds in Britain and Ireland, both in quantity and quality' (Dawson & Allsopp 1986). For two observers on Fetlar, the action started ten days earlier.

Acknowledgment I am most grateful to Dr M. A. Ogttvic for assistance in preparing this paper for publication. References AMERICAN ORNITHOUXASTS' UNION, 1983. The AOU (Jack-list of North American . 6th edn. Iawrence. BYSTRAK, D., & GEISSUVR, P. H. 1986. The breeding bird survey: its first fifteen years, 1965- 1979. US fish & WML Sew. Res. Pubt. 157. DAWSON, I., & AU.SOPP, K. 1986. Recent reports. Brit. Birds 79: 1-17. McIvsREN, 1. A. 1981. The Birds of Sable Island, Nova Scotia. Pmc. Nona Scotia Inst. Set, 31: 1-84. NISBKT, I. C. T. 1970. Autumn migration of the Blackpoll Warbler: evidence for long flight pro­ vided by regional survey. Bird-Banding 41: 238-240. ROBBINS, C. S, i 980, Predictions of future Nearctic landbtrd vagrants to Europe. Brit Birds 73: 448-457. WINGATB, D. B. 1973. A Checkbt and Guide to the Birds of Bermuda. Hamilton. Michael Peacock, Grainel, Gruinart, Bridgend, Isk ofhhy

Peter Lansdown (Chairman, British Birds Rarities Committee) and Dr Alan Knox (Chairman, British Ornithologists' Union Records Committee) have commented as follows: 'A single circula­ tion of the record to each committee was sufficient for the identification of this distinctive species to be accepted unanimously. 'Within the eastern deciduous forests of , this is an abundant species, the third most abundant of 29 warbler species in the breeding bird survey in the eastern (Robbins a al. 1986), comprising about 25% of all Dmdroica species. It was the fourth most abun­ dant of 37 warbler species in TV tower kills in the 1960s (Nisbet 1970), comprising about 8% of all warblers. Despite this abundance, however, it has a somewhat western migration, and is quite scarce as a coastal migrant on the eastern seaboard. For example, at Manomet Bird Observatory, it was only 23rd of 39 warbler species netted in 1966-85, comprising only 0.4% of all catches (MBO Newsletter^ May 1986). On Sable Island, it was recorded in only five of 13 autumns, with a total of eight birds (Mclitren 1981). In Bermuda, it is 'regular but uncommon' in late September, but is outnumbered by at least 19 other warbler species (Wingate 1973). For these reasons, it was not considered a very likely candidate for reaching Europe by Robbins (1980), who calculated a Chestnut-sided Warbler: new to the Western Palearctk 61 predicted likelihood for transatlantic vagrancy for 104 species. It was placed in the bottom group of 35 species with low predicted values. Others from this group, however, including Red-breasted Nuthatch Sitta canadensis, Blackburnian Warbler Dendroica Jusca, Indigo Bunting Passerina cyanea and, most recently, Tree Swallow Tachycineta bicolor, have since reached Europe. Chestnut-sided War­ bler has been recorded in Greenland (AOU 1983). 'The Fetlar Chestnut-sided Warbler fitted the familiar pattern of dates and localities for Nearc- tic Iandbirds in Britain and Ireland. This made the BOURC's task of assessing the Fetlar individ­ ual's likely origin a straightforward one. The Committee voted unanimously for natural occurrence (Brit. Birds 81: 590; Ibis 133: 220), which resulted in Chestnut-sided Warbler being placed in Category A of the British and Irish list.' EDS