American Crow

These images (upper = 21 Mar 2020 at Capitol Lake in Olympia, lower =21 July 2020 at the Nisqually national wildlife refuge) are of the bird formerly known as the Northwestern Crow. Ten years ago, I would have identified these birds as Northwestern Crows, a distinct, small version of the closely related American Crow. Now both these crows are considered to be American Crows. My list of birds seen is down one species. The American Ornithological Society, the group that decides such matters, wrote succinctly that the two former species are to be merged “based on genomic data that indicate a lack of reproductive isolation..., [no] clinal variation, and a lack of consistent differences in size, ecology, and vocalizations.”

DNA research studied DNA from 259 crows across western North America. American and Northwestern crow populations became isolated by late Pleistocene glaciers. During their isolation, coastal populations became smaller than interior ones. When the ice sheets retreated, crow populations again came into contact. Although the populations had changed, they still freely interbred. Today, along their 900 km area of overlap, from Washington across British Columbia, researchers can find no pure American or Northwestern crows. Furthermore, these crows have been hybridizing for a long time, well before modern regional habitat changes. Researchers found pure American Crows in Oregon and pure Northwestern Crows in Juneau, Alaska. All the crows in-between are mixtures between the two.