BOU splits Northern Harrier

Northern Harrier is at  its most distinctive in adult male plumage. Photo: Paulo Philippidis (commons.wikimedia.org).
Northern Harrier is at its most distinctive in adult male plumage. Photo: Paulo Philippidis (commons.wikimedia.org).
After a long wait and several British records, the British Ornithological Union's (BOU) Taxonomic Sub-Committee has announced the split of Northern Harrier from Hen Harrier.

The Northern American equivalent of Hen Harrier – and now a full species in its own right, after being thought a subspecies previously –  has been known to be diagnosable in the field for some years now. Adult male Northern Harriers have reddish-brown spots on the flanks and less black on the primaries among other features, while juvenile and female birds have subtler but still diagnostic characteristics.

Crucially, a 2011 paper on the mitochondrial and nuclear DNA of Circus harriers showed that Northern Harrier was substantially diverged from Hen Harrier, and was in fact more closely related to the Cinereous Harrier of South America.

These now-documented differences in morphology, plumage and genetics leave little option but to split the form as a full species, as  many birders on both sides of the Atlantic have suspected for some time. The famous long-staying juvenile Northern Harrier on St Mary's, Scilly, from October 1982 to June 1983, is now accepted as the first record from Britain. There have also been several occurrences this century.

Other taxonomic changes laid out in the new paper are less relevant to the BOU's British List, but of some general interest. Purple Swamphen has been split into six species, concomitant with well-known plumage differences; the long-standing separation of Azure-winged Magpie into full eastern Asian and Iberian species has been accepted; and another fairly well established split between Crested Lark and its North African analogue – now known as Maghreb Lark – is also supported. 

The widely accepted split between Eastern and Western Subalpine Warblers has yet to be adopted by the BOU, and Moltoni's Warbler is the only former subalpine warbler subspecies listed as a full species in the re-ordering of the genus Sylvia itemised in the paper.

Unfortunately, this is likely to be the last published missive from the sub-committee, as the BOU says it is now disbanding it, after its attempts to unify the taxonomies of the different European rarities Committees ultimately failed. The union will now take on taxonomic decisions directly, while still attempting to unify European lists accordingly.

BOURC. 2016. British Ornithologists' Union Records Committee: 45th Report (October 2015). Ibis 158: 202-205.
Sangster, G, Collinson, J M, Crochet, P-A, Kirwan, G M, Knox, A G, Parkin, D T, and Votier, S C. 2016. Taxonomic recommendations for Western Palearctic birds: 11th report. Ibis 158: 206-212.
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