Nial Moores, Birds Korea, May 26th 2023
Every year in the second half of May, much of the survey focus at migrant hotspots turns toward “grasshopper warblers” (Locustellidae). This group contains several poorly known, highly skulking and similarly-plumaged species. Fortunately, each species has diagnostic, and often distinctive, songs – generally given close to dawn and dusk.
During the morning of May 26th, winds were moderate and from the south, and the sun broke through what little overcast remained from overnight: far from promising for finding many migrants. Because of bus times, I reached Junghwadong in the southwest of Baengyeong Island at about 10 AM, and continued toward the reservoir, en route hearing a couple of song phrases (short, regular, rhythmic – as in this recording here) from a Baikal Bush Warbler Locustella davidi (a fairly rare migrant through the northwest of the ROK) and a few alarm trills and 1-2 song phrases from 2-3 Pallas’s Grasshopper Warbler Helopsaltes certhiola – perhaps the commonest “locust” in spring. It was almost 11:30 when I then heard a much longer, deeper, raspier, crackling buzz of a song, given in bursts of 3 seconds up to 7 or even 8 seconds or so in length. I immediately recognized this song from recordings on Xeno Canto as a Chinese Bush Warbler Locustella tacsanowskia: a species with perhaps only one sight record nationally, and that unsupported by images or sound recordings.
Using a small speaker, I played some Chinese Bush Warbler song phrases which I had downloaded earlier from Xeno Canto. Immediately, the bird moved down the heavily vegetated slope toward me, increasing the frequency of singing in response – and occasionally also giving some very loud, slightly squeeky churring notes. I was able to record the song on my phone, but not these (presumably) aggressive calls. You will need to listen at full volume (best with head phones).
Although the bird stayed very largely hidden, there were short periods when the song was given from slightly more exposed perches within a large bush.
Initial impressions, as shown by the digiscope images, were of a fairly dark grasshopper warbler, with a very pale throat and slightly yellow-washed underparts. It was hard to be certain whether this colour was from reflected light or was genuine saturation. The supercilium was weak but fairly long and, surprisingly, the bird showed obvious spotting on the upper breast. This spotting was a little weaker and thinner, less splotchy-looking than expected in Baikal Bush Warbler, but was far more obvious than I had expected to be shown by Chinese Bush based on hours of online image-browsing in anticipation of encountering one again (remarkably, however, the bird looked closer in plumage – apart from the supercilium – to a drawing of “varient A” in the online Birds of the World by Brian Small than to most of the photographs). The bill was fairly heavy, with prominent orange on the lower mandible – also unexpected based on online accounts.
Fortunately, I managed to take more than one digiscoped image…
And even better – with immense luck and satisfaction – I also managed a short video clip of the bird in song. Please play at full volume to hear a short burst of that amazing song!
If the record is “accepted” (in a nation without a formal review process…) then this presumably becomes the first adequately-documented record of this species in the Republic of Korea, and presumably on the Korean Peninsula as a whole. As such, the species will be elevated from Category 3 to Category 1 of the Birds Korea Checklist during the next revision.
This bird follows on from a sight record of a silent, very plain below Chinese Bush Warbler, also on Baengnyeong Island, on May 25th 2019.
Finally, based on the mapped distribution of this species shown in the online Birds of the World and records in eBird, it seems likely that Chinese Bush Warbler will prove to be a little less rare on the Korean Peninsula than the Baengnyeong records suggest.