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

Bicknell’s thrush (<i>Catharus bicknelli</i>) 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 (<i>Catharus minimus minimus</i> and <i>Catharus minimus aliciae</i>). We recorded NFCs in Milwaukee, US, where <i>C. m. aliciae</i> is the only expected taxon, and compared them with NFCs recorded in Quebec, Canada, where both <i>C. m. aliciae</i> and <i>C. bicknelli</i> occur during migration. We also recorded a small sample of NFCs in Newfoundland, Canada, where the rarer <i>C. m. minimus</i> 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 <i>C. m. aliciae</i>. The second group is characterised by NFCs of much higher frequencies, likely belonging to <i>C. bicknelli</i>. While <i>C. m. aliciae</i> and <i>C. bicknelli</i> appear to produce distinctive NFCs, calls from <i>C. m. minimus</i> were intermediate between these taxa. Based on these results, we provide guidelines for the detection and identification of <i>C. bicknelli</i>.