The spotted thrushes are notoriously difficult to identify. Differences are subtle and subjective, the birds are variable, and it’s often difficult to get a good view of them. On top of that, when we see a thrush it’s usually just that, one bird. It’s difficult to judge subtle comparative differences when there is no direct comparison.
A few field marks are well known and fairly reliable — the contrasting reddish tail of Hermit Thrush, or the bold spectacles of (Olive-backed) Swainson’s Thrush — but species like Gray-cheeked Thrush remain undefined and are often identified more by the absence of field marks.
I’ve been doing a lot of work on thrush ID over the past few years, and have a few preliminary things to share here about the challenge of separating Gray-cheeked and Hermit Thrush. While the reddish tail of Hermit Thrush is a fairly reliable field mark, the tail of Gray-cheeked is also a slightly warmer brown color than the back, and in the field or in photos the difference between the two species is not always obvious. Tail color should be just one of several features you check.
The surest way to distinguish these two species is by wing structure, specifically the emarginations of the outer primaries (scroll to the bottom for details of that). Among all of the spotted thrushes Hermit Thrush has the shortest and most rounded wings, while Gray-cheeked has the longest and most pointed. This is a field mark that is useful in good photos, but very difficult to confirm in live views.
After checking the wing structure, focus on eye ring shape and details of breast spotting and flank color. Using that set of field marks should lead to a confident identification, without even knowing the tail color! The same features – wing structure, eye ring, breast spotting, and flank color – are also the key to identifying the other species in the group.
These are still subtle and subjective features, requiring good views and careful study, but hopefully provide a little more guidance about where to focus your attention when you are lucky enough to catch a glimpse of a thrush.
Hermit Thrush – Eastern
- eyering complete or nearly so, whitish, continuous below eye and often flared slightly in upper rear quarter
- breast spotting continues blackish below breast
- Breast spotting continues onto sides of breast
- spots on breast more rounded, average larger, often running together on upper breast
- lateral throat stripe usually thicker, more continuous
- flanks warmer buff
- wingbars often present
- overall smaller, less streamlined, often more upright posture, crown more rounded
- undertail coverts extend less than halfway along tail
- tail and primary edges distinctly reddish
Gray-cheeked Thrush
- eyering incomplete, whitish, most prominent behind eye, absent or very weak below eye
- breast spotting blackish only on breast, changing abruptly to gray at lower edge of breast and on flanks
- spots on breast more triangular
- lateral throat stripe usually thinner
- flanks more gray-olive, unspotted
- wingbars usually not present
- overall larger, more streamlined, often leaning forward, crown flatter
- undertail coverts extend more than halfway down tail
- tail and primary edges only slightly reddish
Details of wing structure
Hermit Thrush wing structure
- four feathers emarginated, p8 emargination closer to primary coverts than to tip, roughly under middle of secondaries
- short primary projection, shorter than length of tertials
- three longest primaries with p6 nearly as long as 7 and 8, and p9 < p6
- visible primary tips on folded wingtip more or less evenly spaced
Gray-cheeked Thrush wing structure
- two feathers clearly emarginated, p8 emargination closer to tip, well beyond tips of secondaries
- longer primary projection, at least as long as tertials
- two longest primaries with p6 shorter than 7 and 8, and p9 > p6
- last two visible primary tips on folded wingtip longer