By Émile Brisson-Curadeau
As last chronicle rightly points out, the Bicknell’s Thrush is among the rarest and least understood breeding bird species in North America. One major obstacle to its conservation is the difficulty of conducting population surveys. This species spends the summer in the Appalachian Mountains, where access is limited, and winters in the mountains of the Dominican Republic… which are also hard to reach! That leaves migration season—when the thrushes fly over us at night, calling out with vocalizations that are relatively easy to detect. Could we listen for and count these calls to estimate Bicknell’s Thrush populations?
Identification challenge
The problem is that the Gray-cheeked Thrush, a close relative of Bicknell’s Thrush, is not only extremely similar in appearance, but also produces similar calls during its nocturnal migration. Or at least, that’s what we believe—because it’s impossible to visually confirm the identity of the birds in the dark of night.
All we know is that, during the day, both species make similar calls, and those same calls are heard at night while the birds are in flight. So how can we tell which call belongs to which species? That’s exactly the question our study set out to answer.
Biogeography to the rescue
The key to the problem lies in the distribution of the two species. Bicknell’s Thrush has a more restricted range, and everywhere it migrates, the Gray-cheeked Thrush migrates too. This makes it impossible to know which call belongs to which species—at least in those overlapping areas.
However, west of the Great Lakes, only the Gray-cheeked Thrush is present. This means we can use vocalizations recorded in that region as a reference for the species. By comparing those calls to ones recorded farther east—such as in Quebec, where both species migrate—we can look for calls that stand out. If we find consistent differences, we can reasonably suspect that those distinct calls belong to Bicknell’s Thrush!
To test this, we collected hundreds of recordings from both regions and analyzed the spectrograms* of the nocturnal flight calls in an effort to untangle the mystery.
*A spectrogram is a visual representation of a sound, showing how the intensity of different frequencies (in Hertz – Hz) varies over time.
Conclusive results
West of the Great Lakes, calls reach a maximum frequency of around 4600 Hz, with the highest-pitched calls just slightly exceeding 4900 Hz. These values serve as a reference for what a Gray-cheeked Thrush should sound like.
In Quebec, although many calls do cluster around 4600 Hz, we recorded a suspiciously high number of calls exceeding 5000 Hz—and some even reached up to 5800 Hz! These calls could only belong to none other than Bicknell’s Thrush.
The calls of the latter are also, on average, more “explosive.” In other words, the frequency rises rapidly at the moment the call is produced, resulting in a sharp peak on the spectrogram. In Gray-cheeked Thrushes, the frequency increase is more gradual, producing a more rounded shape on the spectrogram. However, caution is needed: some Gray-cheeked Thrushes can also produce explosive calls. The maximum frequency reached therefore remains the best criterion for distinguishing between the two species.
What’s next?
Based on our results, we determined that calls recorded in Quebec exceeding 5000 Hz can be attributed to Bicknell’s Thrush. The next step is to deploy multiple autonomous recording units across the province to capture the passage of Bicknell’s Thrush during migration. By comparing the number of calls recorded over the years, we can begin to track population trends for this elusive species.
Read the full article: Brisson-Curadeau, E., Y. Aubry, A. Desrochers, B.M. Van Doren & B. Drolet. 2025. Identification of nocturnal flight calls of Bicknell’s thrush (Catharus bicknelli) and gray-cheeked thrush (Catharus minimus). Bioacoustics. 1-19. (Download)
