Identification of nocturnal flight calls of Bicknell’s thrush (Catharus bicknelli) and gray-cheeked thrush (Catharus minimus)

Bicknell’s thrush (Catharus bicknelli) is a rare North American songbird that may be best detected via its nocturnal flight call (NFC) during seasonal migration. However, there is debate around whether its NFC can be reliably separated from that of the two grey-cheeked thrush subspecies (Catharus minimus minimus and Catharus minimus aliciae). We recorded NFCs in Milwaukee, US, where C. m. aliciae is the only expected taxon, and compared them with NFCs recorded in Quebec, Canada, where both C. m. aliciae and C. bicknelli occur during migration. We also recorded a small sample of NFCs in Newfoundland, Canada, where the rarer C. m. minimus breeds. Using unsupervised Gaussian mixture models, we found that the Quebec dataset can best be explained by a combination of two groups: the first group is identical to the NFCs collected in Milwaukee and is consistent with C. m. aliciae. The second group is characterised by NFCs of much higher frequencies, likely belonging to C. bicknelli. While C. m. aliciae and C. bicknelli appear to produce distinctive NFCs, calls from C. m. minimus were intermediate between these taxa. Based on these results, we provide guidelines for the detection and identification of C. bicknelli.