Sunday, May 22, 2016

Identification of a Gray-cheeked Thrush

Field Encounter
On May 8, 2016, Ronnie d'Entremont found a thrush lacking the chestnut tail of a Hermit and the buffy face of a Swainson's. This left him with two choices: Gray-cheeked and Bicknell's. He sent photos to Mark Dennis who felt that they best fit Gray-cheeked. By the end of the day, a few of us were able to get great views of the bird along with unobstructed photos.

Taxonony & Occurence
It was only recently when Ouellet (1993) concluded that Bicknell's and Gray-cheeked thrushes were distinct species. These two were previously considered conspecific as Gray-cheeked Thrush, consisting of three subpecies: the present day Catharus minimus minumus (Gray-cheeked of Newfoundland and n. Quebec) and C. m. aliciae (Gray-cheeked breeding from Alaska to Labrador) along with Bicknell's Thrush (Catharus bicknelli). The split was accepted by the American Ornithologists' Union in 1995.

Both Bicknell's and Gray-cheeked occur in Nova Scotia during migration, but breeding has only been confirmed with Bicknell's, mainly in the Cape Breton Highlands and formerly on our south shore islands (McLaren 2012). The second Maritime Breeding Bird Atlas found that Gray-cheeked Thrushes were probably breeding on Harbour Island, Halifax County and White Head Island, Guysborough County (Stewart et al. 2015).

Many sources state that differentiating between silent Gray-cheeked Thrush and Bicknell's Thrush in the field is difficult (Sibley 2014, Alderfer 2014...). The good quality photos of this bird in multiple positions are required in order to attempt confident identification of a silent bird. Lane & Jaramillo (2000) suggest that a broadside view under uniform lighting are required to make an accurate judgement of minute differences in colour. This was the case with this thrush as it was in the shade among willow trees producing diffused lighting. With perfect conditions and crisp photos from various angles, we have the best material to work with for a proper identification.

Primary Bases
The colour contrast between the bases of the primaries and the rest of the wing are claimed as a useful in field identification of the two species (Lane & Jaramillo 2000). In Bicknell's, the primary bases are slightly warmer and more reddish-brown than the rest of the wing. In typical aliciae Gray-cheeked, the bases are paler and appear more washed out graying-brown, while C. m. minimus have warmer brown primary bases which may be indistinguishable from Bicknell's. Figure 1 provides a nice view of the primary bases, which appear to be a paler brown than the rest of the wing without any appreciable reddish-brown, but perhaps slightly warmer than the remainder of the wing.

Underparts Colour
The flanks of Bicknell's are described by Lane & Jaramillo (2000) as reflecting the warmer brown upperparts seen in this species while contrasting more with the white belly than the washed-out, grayer flanks and belly of a Gray-cheeked. It is important to note that C. m. minumus tends to show warmer flanks than does C. m. aliciae. The contrast between cold-coloured flanks and belly (Figs 1 & 2) on the Chebogue Thrush seems minimal, suggesting Gray-cheeked.

Figure 1. Chebogue Thrush at Chebogue Point, Yarmouth County, May 8, 2016. A thrush with little eye ring, cold overall plumage and long primary extentions past the tertials. Photo by Alix d'Entremont.

Figure 2. Chebogue Thrush at Chebogue Point, Yarmouth County, May 8, 2016. The flanks appear to be less brownish than in Bicknell's and don't contrast very much with colour of the belly. Photo by Alix d'Entremont.

Upperparts Colour
Lane & Jaramillo (2000) state that the upperparts of Gray-cheeked are a monotone, cool olive-brown or grayish brown, while Bicknell's varies from a dull brown to a warm olive-brown, almost chestnut on the tail. The uppertail coverts and tail of Bicknell's is said to be a richer chestnut-brown compared to that of the Gray-cheeked that are concolorous with the back, however C. m. minimus approaches Bicknell's.

Luckily there was a Hermit Thrush at the exact same location, so useful colour comparisons can be made. It is clear that the Hermit Thrush (Figure 4) is much warmer overall and shows a clearly bright chestnut tail. Compare this to the extremely cold-looking Chebogue Thrush and you get an appreciation of how little warm browns and reds are apparent its overall plumage. Figure 3 clearly shows that the cold-coloured back of the Chebogue Thrush contrasts with the warmer tail, more reminiscent of either Bicknell's or minimus Gray-cheeked.

Figure 3. Chebogue Thrush at Chebogue Point, Yarmouth County, May 8, 2016. This photo provides a nice view of the warmer-toned tail in comparison to the more grayish-brown back. Photo by Alix d'Entremont.

Figure 4. Hermit Thrush at Chebogue Point, Yarmouth County, May 8, 2016. Note how cold-toned the upperparts of the Chebogue Thrush are compared to this Hermit Thrush.  Photo by Alix d'Entremont

Wing Ratio
While plumage and bare parts colouration are suggestive, Townsend et al. (2015) explain that vocalizations and morphometrics are much better tools for identification. Since this bird was silent, we must rely most on shape captured in the photos. Lane & Jaramillo (2000) propose that the ratio of the length of the primary extension beyond the tertials to the length of the exposed tertials might be the best way to compare the wing length of both species. They found that in Bicknell's, this ratio is 1:1 or less while in Gray-cheeked, it is 1:1 or greater. I have measured these dimensions on two photos of the thrush in question using Adobe Photoshop. The measurements in pixels as well as the resultant ratios are given below.

Exposed tertials: 243
Primaries: 311
Ratio: 1.28:1

Exposed tertials: 230
Primaries: 278
Ratio: 1.20:1

We see that the ratio matches that expected for Gray-cheeked, a result of that species' long wings. A Bicknell's Thrush was recently photographed in New Jersey and the photos allow for measurement of this ratio and produces a value of about 1:1. While the New Jersey bird doesn't show a lot of contrast between the back and tail, the upperparts are clearly warmer than that of the Chebogue Thrush. The pale base of the lower mandible of that Bicknell's is more extensive (far past the nostrils) than that of the Chebogue Thrush and seems to have a more yellow, approaching orange colouration and less fleshy tones than the Chebogue Thrush. Here, and here are a few more photos of the New Jersey Bicknell's.

Wing Morphology
Another wing feature that provides evidence of this bird's identification as a Gray-cheeked is the wing morphology. Pyle (2007) illustrates how P7 & P8 are of equal length in the folded wing in Bicknell's while P8 is longer than than P7 in Gray-cheeked. Figure 5 shows the relative length of the primaries, note how P8 is slightly longer than P7 and how P9 is hidden under P8, just like Pyle's description of Gray-cheeked.

Figure 5. Chebogue Thrush at Chebogue Point, Yarmouth County, May 8, 2016. This composite shows the wing morphology which matches Gray-cheeked more than Bicknell's. Photo by Paul Gould.

Ouellet (1993) states that Bicknell's Thrush's bill base is bright pale yellow, whereas it is flesh or yellowish flesh in Gray-cheeked, and Todd (1963) noted that C. m. minimus of Newfoundland had a more extensive pale base than C. m. aliciae and Townsend et al. (2015) quantify this by describing that the pale extent in minimus extends beyond the nostril. The pale base to the mandible on the Chebogue Thrush pushes slightly beyond the nostril, enough to consider minumus. The colour of the pale base seems to show fleshy tones, but appear similar to some Bicknell's photos on the internet. Lane & Jaramillo (2000) do conclude that both the colour and extent of the pale base of the mandible is unreliable, so little weight will be given to it here.

Migration Timing
eBird reports along with migration timing data provides evidence in support of Gray-cheeked Thrush. As of May 8, all of the reports of Bicknell's Thrush are limited to the Dominican Republic and Puerto Rico. In contrast, reports of Gray-cheeked Thrush come from as near as New York state. The earliest arrival date of migrant Bicknell's in New England as reported by Townsend et al. (2015) is May 15. McLaren (2012) does mention a report of Bicknell's on April 23 1983 at Brier Island, but this was during an extraordinary event where numerous thrushes arrived extremely early. There was no similar event in this case, so the arrival of a Bicknell's Thrush by May 8 excluding fallouts is unlikely.

ID Conclusions
We have strong morphological evidence that the Chebogue Thrush is a Gray-cheeked. I don't feel confident in assigning this bird to a subspecies with any great amount of certainty. The back and tail of the Chebogue Thrush are not concolorous, more similar to that of minimus, but the primary bases don't appear to have any reddish-brown. The extent of the pale base of the mandible also seems within range for minimus. Due to these features, I would lean towards this being a Gray-cheeked Thrush headed to Newfoundland or northern Quebec (Catharus minimus minimus).

Alderfer, J., J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.

American Ornithologists' Union. 1995. Fortieth supplement to the American Ornithologists' Union Check-list of North American Birds. Auk 112: 819-830.

Lane, D. F. and A. Jaramillo. 2000. Identification of Hylocichla/Catharus thrushes; Part III: Gray-cheeked and Bicknell’s thrushes. Birding 32(4): 318-331.

McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada

Ouellet, H. 1993. Bicknell's Thrush: Taxonomic status and distribution. Wilson Bulletin 105: 545-754.

Pyle, P., S.N.G. Howell, R.P. Yunick, and D.F. Desante. 1997. Identification guide to North American Birds, Part 1, Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.

Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.

Stewart, R.L.M., K.A. Bredin, A.R. Coururier, A.G. Horn, D. Lepage, S. Makepeace, P.D. Taylor, M.-A. Villard, and R.M. Whittam (eds.). 2015. Second Atlas of Breeding Birds of the Maritime Provinces. Bird Studies Canada, Natural History Society and Prince Edward Island, Nature New Brunswick, New Bunswick Department of Natural Resources, Nova Scotia Bird Society, Nova Scotia Department of Natural Resources, and Prince Edward Island Department of Agriculture and Forestry, Sackville, 528 + 22 pp.

Todd, W.E.C. 1963. Birds of the Labrador Penunsula and Adjacent Areas. Toronto: University of Toronto Press.

Townsend, Jason, Kent P. McFarland, Christopher C. Rimmer, Walter G. Ellison and James E. Goetz. 2015. Bicknell's Thrush (Catharus bicknelli), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:


  1. Superb analysis and presentation Alix

  2. What an incredibly useful write-up. I saw what I presumed was a GCTH (based on the gray overall dull colour)sitting on the top of an open outbuilding door on BPI in 1990. No camera! Your photos here are excellent in support of your text. For interest, among my miscellany, I have a note that in the 1884 Volume 1 of The Auk that a J.H. Langille published an account of a Bicknell's Thrush found Breeding on Seal Island. How I wish that Sylvia Fullerton, who found a GCTH on Seal in 2014, had photos. A few of us in the group saw the bird less than an hour later, but in very poor light. I presumed GCTH again, but I see now that this is not an easy bird to ID. Many thanks, Alix.