Background Circus cyaneus is a medium-sized bird of prey that is widely distributed across the Northern Hemisphere. There are two currently recognized forms, the Palearctic form C. c. cyaneus (Hen Harrier), and the Nearctic form C. c. hudsonius (Northern Harrier). The forms have recently been split by the British Ornithologists' Union but the American Ornithologists' Union and some other taxonomic committees have not yet made any change. Here we examine the phylogenetic relationship between the two forms using sequence data from multiple nuclear and mitochondrial genes and examine breeding biology, body size, morphology, dispersal and other behaviors.

Methods In order to fully compare cyaneus and hudsonius, we carried out a full literature review, measured museum skins and carried out phylogenetic analysis using a number of different mitochondrial genes and compare our findings to other recent work.

Results We find that these two allopatric taxa form reciprocally monophyletic groups, show substantial mtDNA sequence divergence, and further differ significantly with respect to body size, plumage characters, breeding biology, dispersal and other behavioral traits.

Conclusions Based on an array of consistently divergent characteristics, it is suggested that the two forms are best regarded as separate species, Hen Harrier (Circus cyaneus) and Northern Harrier (Circus hudsonius).