Bicknell’s Thrush Catharus bicknelli

Bicknell’s Thrush was long-considered a subspecies of Gray-cheeked Thrush, but when the AOU split Gray-cheeked Thrush and gave Bicknell’s full species status in 1998 it created one of the most challenging field identification problems in eastern North America.

As a broad generalization Bicknell’s Thrush is smaller and more reddish than Gray-cheeked. The real problem is that the two species overlap in all visible features, including size and color. The study that led to the split (Ouellet, 1993) documented this overlap, and I noticed the same issue in my own studies. Song is the best way to distinguish the species.

Identification: Bicknell’s vs. Gray-cheeked Thrush

• Song (see below)
• Face more uniform and more reddish brown
• Overall color more reddish on average, especially on tail and primaries (vs. more grayish)

The western subspecies of Gray-cheeked (subspecies aliciae, nesting from Ontario to Alaska) averages large and grayish and these birds are fairly distinctive in both color and size, but the eastern subspecies of Gray-cheeked Thrush (subspecies xx nesting from Quebec to Newfoundland) averages smaller and redder, overlapping extensively with Bicknell’s. Not only do color and size overlap, but these two variables are not entirely linked. That is, there is a cline in color from reddish to gray and a cline in size from small to large, but some birds are small and grayish while others are relatively large and reddish. The two museum collections I have looked at ((specimens at Philadelphia Academy of Natural Sciences and Yale Peabody Museum)) show a confusing array of size and color.

• Overall size smaller on average

Size is difficult to judge and with extensive overlap, this is not very reliable and is only useful as a general impression to help pick out a potential Bicknell’s. Bicknell’s might appear more pot-bellied, less sleek and sturdy than Gray-cheeked (reminiscent of Hermit Thrush) but this also is just an impression.

• Primary projection shorter on average
• Base of lower mandible more yellow (vs more flesh-colored)

This has often been mentioned in the literature as a distinguishing feature, and I’ve never been sure whether the claim is that the color is more yellow, or the yellow more extensive, or both. In any case it seems too variable and too subjective to be much use in the field.

Limitations of identifying by impressions

Impressions of shape and size can be helpful, as the “classic” Bicknell’s is smaller, shorter-winged, more dumpy and Hermit-Thrush-like in shape compared to the “classic” Gray-cheeked. But by relying on that one is likely to misidentify some smaller and redder Gray-cheekeds and also to overlook larger and grayer individual Bicknell’s. It is an oversimplification to suggest that these species can be distinguished by impressions of shape and color.

I would also caution against the “You’ll know it when you see it” school of identification. A statement such as “When I see a Bicknell’s it just jumps out as redder and less robust” is a classic example of what psychologists call a “one-sided event”. Only the birds that seem most obviously reddish and/or small will attract attention and be identified. Bicknell’s Thrushes that are not quite so reddish or small are simply not noticed, so it is predetermined that the hypothesis will be “confirmed”. This sort of vague impression of differences is also subject to illusions of color (see my post here), and some Gray-cheeked Thrushes can momentarily seem very reddish or very small. Not every Bicknell’s will “jump out”, and not every bird that “jumps out” is a Bicknell’s.

Voice (and primarily song) is still the most reliable way to distinguish these species. In areas where they are expected, it may be reasonable to identify small and reddish birds as Bicknell’s Thrush after careful and lengthy study, even without hearing them sing, but I don’t think sight observations or photos alone would ever be acceptable evidence of a bird out of range, for example, in Ohio, or Alaska, or the UK.

Voice

Song of Bicknell’s Thrush is fundamentally similar to Gray-cheeked, and differs mainly in a few details of inflection as described in the Sibley Guide. Both can be distinguished from Veery and Swainson’s Thrush by the somewhat thin and nasal quality of the sound, and from Swainson’s by the descending trend in pitch of the whole song.

Flight calls also differ slightly, although this difference is so subtle that few observers will have the experience necessary to use it.

Questions:

• It has been reported that Bicknell’s sometimes sings songs like Gray-cheeked. I can believe this is true but it needs confirmation and further study.
• Bicknell’s apparently sings an alternating series of different songs, with each male having three or four songs in its repertoire. This is unlike Gray-cheeked Thrush, which apparently have a single song in their repertoire and sing that over and over. If true and consistent this could be another useful clue to identify a singing Bicknell’s Thrush.

References and more info:

Ouellet, H. 1993. Bicknell’s Thrush: Taxonomic Status and Distribution. Wilson Bull., 105(4), 1993, pp. 545-512. pdf here: http://elibrary.unm.edu/sora/Wilson/v105n04/p0545-p0572.pdf

The International Bicknell’s Thrush Conservation Group  website: http://www.bicknellsthrush.org/