Subspecies of the Dark-eyed Junco (Junco hyemalis) differ based on morphology and genetics, with interbreeding occurring where subspecies meet and overlap. Two subspecies breed in eastern North America: J. h. hyemalis and J. h. carolinensis. They have a historical intergrade zone between Pennsylvania and southern New York. Based on range shifts in other bird species in recent decades, we considered that the southern carolinensis subspecies may have shifted northward as well. We compared morphology of more than 300 breeding and overwintering juncos from western Massachusetts to known carolinensis and hyemalis populations to determine what subspecies is currently found in our study area. Based on bill color, plumage characteristics, and morphological comparisons of wing chord, tarsus length, bill length, and tail white, we concluded that the hyemalis subspecies makes up the breeding and overwintering populations in western Massachusetts and that carolinensis has not moved into western Massachusetts. Our study provides valuable baseline measurements on junco morphology in order to detect future shifts in junco subspecies intergrade zones in the northeastern United States.

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