«Grey-cheeked and Bicknell's Thrushes: taxonomy., identification and the British and Irish records Alan Knox The BOU Records Committee has recently ...»
Established 1907; incorporating 'The Zoologist', established 1843
Grey-cheeked and Bicknell's
Thrushes: taxonomy., identification
and the British and Irish records
The BOU Records Committee has recently accepted
the taxonomic split of the mainly Nearctic Grey-cheeked Thrush
Catharus minimus into two species: a restricted Grey-cheeked
Thrush comprising the races minimus and aliciae and the
monotypic Bicknell's Thrush C. bicknelli. Identification and ageing of the three forms is discussed.
The races aliciae and minimus are both larger than bicknelli, but measurements overlap. The plumage of aliciae is colder and greyer than the generally warmer and more rufous minimus. In this respect, minimus approaches bicknelli. Despite further differences in biometrics, vocalisations and bare-part colours between Grey-cheeked and Bicknell's Thrushes, the field identification of most migrants is not recommended at this stage.
The British records of 'Grey-cheeked Thrush' (in the wide sense) are reviewed. The single previously accepted record of bicknelli is no longer considered to be acceptable, and the form has been deleted from the British & Irish List.
This paper is an official publication of the British Ornithologists' Union.
1 [Brit. Birds 89: 1-9, January 1996] © British Birds Ltd 1996 2 Knox: Grey-cheeked and Bicknell's Thrushes The Grey-cheeked Thrash Catharus minimus (as it is currently known) breeds in the boreal coniferous forests of North America and northeastern Siberia. The Nearctic distribution covers Alaska and northern Canada from the Yukon to Newfoundland and northern Nova Scotia and, in the mountains, south through New England.
Taxonomy and geographic variation
The species is usually divided into three races (Godfrey 1986; Ouellet 1993):
bicknelli has a restricted distribution and breeds on the mountains of eastern New York State, New England, southern Quebec and the Canadian Maritime Provinces;
minimus also has a restricted distribution, breeding only in Newfoundland, southern Labrador and extreme eastern Quebec;
aliciae nests over the remainder of the species' range, from Labrador to eastern Siberia.
Cramp (1988) followed the AOU (1957) in regarding aliciae as indistinguishable from minimus. Todd (1963) had, however, shown that aliciae is recognisable, a view shared by most subsequent authors (e.g. Godfrey 1986).
The three forms are rather similar in appearance. Nominate minimus and aliciae tend to be larger than bicknelli (table 1), but measurements overlap. The upperparts are dark olive-grey on aliciae, and browner on minimus. In bickneM, the upperparts tend to be richer brown than those of minimus, especially on the tail. The plumage of all three forms is, however, quite variable (Wallace 1939;
It has recently been suggested by Dr Henri Ouellet that the southern, New England and Maritime Province race should be treated as a separate species C.
bicknelli, Bicknell's Thrush, with nominate minimus and aliciae remaining together as the Grey-cheeked Thrush C. minimus (Ouellet 1993). Ouellet showed that Bicknell's and Grey-cheeked Thrushes differ in several respects: size and, to a certain extent, plumage and bare-part colour; the breeding and wintering ranges of the two species do not overlap; they have different songs (and nocturnal flightcalls: Evans 1994), and playback experiments on the breeding grounds have shown that Bicknell's Thrushes do not respond to recordings of minimus or aliciae; there are differences in the breeding habitats of Bicknell's and Greycheeked Thrushes; there is no known hybridisation or intergradation; and analysis of mitochondrial DNA from Bicknell's and Grey-cheeked Thrushes has revealed sequence divergence (1.7%) which is greater than that of many avian sibling species (Ouellet 1993).
Following the American Ornithologists' Union (in press), the British Ornithologists' Union Records Committee (in press) has recently accepted that Bicknell's and Grey-cheeked Thrushes should be treated as separate species.
Field identification Separation of migrant Grey-cheeked and Bicknell's Thrushes is complicated by the geographic variation within the former. Nominate minimus is noticeably browner and richer than aliciae. If individuals of minimus and aliciae were seen alongside one another, the former might be mistaken for bicknelli. Indeed, there 3 British Birds, vol. 89, no. 1, January 1996 is a closer resemblance between some minimus and bicknelli than between minimus and the greyer and more olive aliciae.
There are several drawers containing skins of Grey-cheeked Thrushes at the Natural History Museum at Tring, but these include only a few breeding birds, hardly any correctly identified autumn bicknelli, and most of the non-breeding minimus and aliciae specimens are difficult to separate. To supplement the Tring material, I borrowed a short series of each of aliciae, minimus and bicknelli from the American Museum of Natural History. Using these together with the Tring specimens, I was able to confirm the essentials of the assessment of aliciae by Todd (1963).
UPPERPARTS, INCLUDING TAIL Nominate minimus is a lighter, warmer brown than the colder, darker, olive-brown or greyish-olive of aliciae. The edges to the primary coverts, greater coverts and primaries on the folded wing tend to be warmer and lighter on minimus, and contrast more with the rest of the upperparts and the dark tips to the primary coverts in particular.
UNDERPARTS On minimus, the spots on the breast tend to be lighter and less contrasting and, although there is much variation, the ground colour is often warmer. The flanks are also richer than the cooler grey of aliciae.
BARE PARTS Todd (1963) noticed that the pale area at the base of the lower mandible was larger on minimus than on aliciae, and, although there was much variation, this was also generally the case in the series available to me.
All these differences are best seen in series of birds rather than on individuals.
Differences between Grey-cheeked and Bicknell's Thrushes Grey-cheeked and Bicknell's Thrushes are very alike. The greatest difference is in size, but measurements overlap, and size is unlikely to be helpful in separating the species in the field (table 1). In all other characters, there is considerable variation, and the differences between the species are not great. The tail is often more rufous on Bicknell's in breeding plumage, but this is less obvious in the autumn when the feathers are fresh (Ouellet in litt.). There are minor differences in bare-part colours. On Bicknell's Thrushes, the base of the lower mandible is bright, pale yellow. On Grey-cheeked Thrushes, the pale area at the base of the lower mandible tends to be less extensive and more typically flesh-coloured or yellowish-flesh, although some have uniformly horn-coloured bills. The legs of Bicknell's Thrush tend to have a purplish wash rather than the browner (horn) colour of Grey-cheeked Thrush, but this would be difficult to see in the field or to judge on lone birds. Similarly, the brighter yellow soles to the feet of Bicknell's Thrush are unlikely to help unless birds are trapped. Even so, comparison with known colours would be desirable.
Although there are differences in the song and the calls of Grey-cheeked and Bicknell's Thrushes, these are of no use with non-vocal autumn migrants. Within each of the two species, there is sufficient variation in plumage and bare-part colours to make it difficult or inadvisable, with present knowledge, to identify single autumn migrants in the field. The exception to this might be that very grey birds with restricted, flesh-coloured bases to the lower mandibles are almost certainly Grey-cheeked Thrushes of the race aliciae.
4 Knox: Grey-cheeked and Bicknell's Thrushes
The identification of Grey-cheeked and Bicknell's Thrushes was recently discussed in detail by Curson (1994). Unfortunately, in presenting Ouellet's data, Curson did not mention that Ouellet's study was carried out mainly on breeding birds. Worn summer specimens are substantially different in appearance from autumn birds in fresh plumage. Furthermore, Curson did not discuss (or even mention) aliciae, and this seriously undermines the usefulness of the article, where almost every reference to differences between Grey-cheeked and Bicknell's Thrushes applies similarly to differences between aliciae and minimus.
The difficulties of interpreting subtle colours in photographs continue to be underestimated (Knox 1993, page 365). Photographs used by Curson (1994) to illustrate differences between Grey-cheeked and Bicknell's Thrushes actually provide examples of such photographic effects (e.g. plates 1-3 are of the same individual by the same photographer: in plate 2, the breast and upperparts are a warm brown shade, whereas in plate 3 they are cold olive; similarly, plates 4 & 5 are of another individual and again the colours are noticeably different in the two photographs; plates 6, 8 & 9 are of a claimed Bicknell's Thrush: plate 6 shows a bird with a warm hue to its plumage, in stark contrast to the colder, greyer plumage in plate 8; the colour of the bird in plate 10 is unlike any Grey-cheeked Thrush, yet colours are discussed in detail). The captions to these photographs and the editorial following the article fail to address these problems realistically.
British Birds, vol. 89, no. 1, January 1996 5 For further examples of differences between photographs of an individual Grey-cheeked Thrush, see Brit. Birds 85: 542-543, 568, plates 236, 239, 251 & 252.
Ageing First-years often have conspicuous pale tips to the greater coverts, but not all those birds with uniform coverts are adults. Several specimens of all three forms which I aged as first-winters on the basis of the pointed shape of their tail feathers had uniform coverts. Pyle et al. (1987) noted that about 15% of first-years had plain greater coverts. Whereas individuals with clear spots on the coverts may safely be aged as first-years, those with plain coverts are more reliably aged by the shape of the tail feathers (see Pyle et al. 1987, McMinn 1995). According to Wallace (1939), first-years tend to have browner wing-panels.
Vagrancy potential With its large range and northerly distribution as far east as Labrador, aliciae is a likely vagrant to Britain and Ireland. Nominate minimus, breeding as it does in Newfoundland, starts its migration with a flight over open sea and is also a candidate for storm-diverted or ship-assisted passage. Given the more southerly range of bicknelli and its status as 'potentially endangered' (Faccio 1995), Bicknell's Thrush may be less likely than either of the other two forms to occur as a vagrant in Europe, although it has occurred in Bermuda (Ouellet in litt.).
British and Irish records The first British Grey-cheeked Thrushes were trapped on Fair Isle, Shetland, on 5th October 1953 (Williamson 1954) and 29th October 1958 (Davis 1959). In all, there has been a total of 43 Grey-cheeked Thrushes in Britain and Ireland to the end of 1994 (Fitzharris 1983; Rogers et al. 1995; Smiddy & O'Sullivan 1994).
Of these, no fewer than 17 have been trapped, were found dead or died later (table 2).
Present location of the dead birds
3. BARDSEY, 1961 It was particularly important to find this specimen as it is the only one said to have been identified as bicknelli (by Charles Vaurie, in Clafton 1963). This bird was skinned on the island and later sent to Vaurie at the American Museum of Natural History. It was returned and apparently travelled around before being given to a museum (Frank Clafton verbally). After several months of searching, I was unable to locate the specimen and it may now be lost.
It is apparently not at the National Museum of Wales at Cardiff, the University College of North Wales at Bangor, nor at Liverpool, Manchester or Bolton Museums nor at the Natural History Museum at Tring.
4. ST KILDA, 1965 Now at the Royal Museum of Scotland in Edinburgh, no.
5. LOSSIEMOUTH, 1965 Now at the Harrison Zoological Museum, Sevenoaks, no.
6 Knox: Grey-cheeked and Bicknell's Thrushes
6. HORDEN, 1968 This one was said to be at the Hancock Museum in Newcastle (Brit. Birds 62: 476-477). The museum, however, has no record of the bird.
It later emerged that the corpse had been given to the museum and prepared as a freeze-dried specimen, which was subsequendy eaten by a rat (Eric Meek verbally).
8. BARDSEY, 1971 Said to have been going to the National Museum of Wales (BBRC submission), this bird was eventually traced to an outbuilding in Dyfed.
It has now been deposited at the N M W, Cardiff, no. N M W Z. 1995.014.
li. ST MARY'S, 1986 This bird attracted the attention of a cat. I am not aware of the existence of either measurements or identifiable surviving parts.
12. ST MARY'S, 1986 Mounted specimen in the possession of Chris R. Janman.
13. LUNDY, 1986 Remains of dead bird found, but apparently not retained.
15. SLIMBRIDGE, 1990 T h e skin was said to have been retained by the Wildfowl & 7 British Birds, vol. 89, no. 1, January 1996 Wetlands Trust (Brit. Birds 84: 485). On investigation, it was discovered that the specimen rotted during a freezer failure and was lost prior to skinning (M. Brown verbally).
Having straggled to get across the Atlantic, at least nine of the 43 Greycheeked Thrashes died shortly after arrival. In general, we appear to have been poor custodians of their earthly remains. Observers finding the bodies (or parts, even single feathers) of rare birds are strongly urged to deposit them as soon as possible for safe keeping in the national collections at Tring, Cardiff, Liverpool or Edinburgh.
Identification of the existing specimens The thrushes which were traced were compared with skins at Tring and the specimens on loan from the AMNH. All four British birds are Grey-cheeked Thrushes rather than Bicknell's. Based on the limited material available, the birds from Lossiemouth (no. 5), St Mary's (no. 12) and probably St Kilda (no. 4) seem closest to aliciae and the Bardsey specimen (no. 8) may be closer to minimus. The originally published wing-length of the Lossiemouth bird was presumably a typographic or transcription error (see table 3).