Harriers

During Audubon’s time all the way until the mid-twentieth century, it was considered a separate species (Marsh Hawk – Circus Hudsonius) from the similar harrier found throughout Eurasia (the Hen Harrier – Circus cyaneus). Then sometime between the 1930’s and the 1950’s, its species became considered conspecific with the Hen Harrier – Circus cyaneus, but we still called it the Marsh Hawk. In 1982, the American Ornithologists Union decided to change its common name to match the rest of the birds in the genus Circus*. So they renamed it Northern Harrier. 

A genetic study in 2015 showed that the Northern Harrier in America was actually more closely related to the Cinereous Harrier (Circus cinereous – whose range is in South America just south of the southernmost winter range of the Northern Harrier) than the Hen Harrier of Eurasia. So that is why the A.O.U. removed the Northern Harrier from Circus cyaneus and returned it to its original species Circus hudsonius. However, they still want to keep the name Northern Harrier. The common name of the Circus cyaneus harrier in Eurasia is returned to Hen Harrier.