(noun) A bird that is notably different from the expected characteristics of its species, but not sufficiently different to be independently classified as a unique species. Subspecies are often geographically determined, and differences such as size variations or plumage colorations are a common basis for subspecies distinctions.
Not all bird species have distinct subspecies, and over time the classifications of different birds can change to alter subspecies into a lesser or greater number of divisions, or even to grant a subspecies distinction as a new bird species. Examples of common subspecies include the different geographic plumage variations of the dark-eyed junco, such as the eastern "slate-colored" junco and the western "Oregon" junco, as well as the southwestern subspecies of the lesser goldfinch, the "black-backed" lesser goldfinch.
While a subspecies can be noted on a birder's life list, it does not count as an additional bird for a life list total unless the bird is reclassified as a distinct species. Many birders enjoy the extra challenge of seeing different subspecies, and comprehensive field guides will list common subspecies. These details also make a field guide more useful over time as bird classifications may change.
Photo – "Oregon" Dark-Eyed Junco © Brendan Lally