Taxonomy and distribution of the imperilled Newfoundland Gray-cheeked Thrush, Catharus minimus minimus

Gray-cheeked Thrushes breeding on Newfoundland are purported to be a distinct subspecies (Catharus minimus minimus) and have declined precipitously since the 1980s. To assess the validity of Gray-cheeked Thrush subspecies we collected blood samples and morphological measurements from 51 individuals captured at 15 sites in Newfoundland and Labrador (2013-2015). Analysis of mitochondrial (ND2) and nuclear intron (ADAM-TS 6, FIB7) sequences from these and additional samples from Nova Scotia, Newfoundland, Labrador, Quebec, Alaska, and Siberia showed low genetic variation at both nuclear loci, and shallow mitochondrial divergence between subspecies; there were no shared haplotypes between thrushes from Newfoundland / Nova Scotia (n = 41) and those from western Labrador and further west (n = 24). Thrushes from Newfoundland also had shorter wing chords, tails, and culmens and less black in the mandible compared to those from western Labrador and Quebec. Samples from the southeast coast of Labrador (n = 13) included ND2 haplotypes both from Newfoundland and western Labrador plus one putative hybrid that was phenotypically a Gray-cheeked Thrush but that had a Bicknell's Thrush (C. bicknelli) ND2 haplotype and was heterozygous at a segregating site in FIB7. We detected thrushes during point counts at 7 of 24 sites on Newfoundland, but failed to detect them at 10 historically occupied sites on Newfoundland or in the reported distribution gap between subspecies in Labrador. Sites where thrushes have apparently disappeared had less shrub habitat within 1250 m and more large broadleaf trees within territory-scale areas compared to sites where they persist. Additionally, red squirrels (Tamiasciurus hudsonicus) are an introduced species on Newfoundland and thrush occurrence was > 3x higher at sites where red squirrels were not detected. Our results support previous designations of C. m. minimus from Newfoundland and southeastern Labrador as a subspecies distinct from C. m. aliciae found further west.