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Citation

Dobson A & Clarke M (2011) Inconsistency in the taxonomy of Hen and Northern Harriers: Causes and consequences. British Birds, 104 (4), pp. 192-201. http://www.britishbirds.co.uk/articles/inconsistency-in-the-taxonomy-of-hen-and-northern-harriers-causes-and-consequences

Abstract
The Hen Harrier and Northern Harrier are classified as conspecifics by some modern authors and as separate species by others. The tendency to treat the two as conspecifics may have its origin in erroneous nineteenth-century illustrations, and their allopatry has meant that twentieth-century adherence to the Biological Species Concept has hindered attempts to classify them to universal approval. Andrew Dobson and Michèle Clarke tell how this difference of opinion is a hindrance to our understanding of both harriers, and may adversely affect their conservation.

Journal
British Birds: Volume 104, Issue 4