'This bird appears to tick all the boxes for the American race of hen harrier (split by some authorities as Northern harrier, but not yet by the BOU), a taxon which has occurred just three times before in Britain and never in Cambs (an old claim from Wicken Fen was found not proven). The unstreaked bright orange underparts and dark hood separate it from some lookalike cyaneus hen harriers, which can look very similar though generally possess some obvious streaking on the breast, flanks and/ or vent.
There are a range of subtle features all of which support the identity as that of a hudsonius, falling outside the known range of variability of cyaneus, including in particular the underwing pattern (especially the number of bars on the outer primaries) but criteria for acceptance is very much a work in progress and the goal posts could shift as we learn more about the taxon. It was important to sex this bird as there are minor differences between juveniles of the two sexes and the dark brownish eye colour indicates this one is a female. Rather oddly the form has recently started occuring with some regularity at Tacumshin, Co Wexford in Ireland, prompting some to question its true status and consider whether Northern harriers could be hybridising with cyaneus hen harriers somewhere. A DNA sample was recently obtained from one of the Irish birds and this may help to clarify the situation.' JH
For further discussion, see http://jameshanlonbirder.blogspot.co.uk/2013/10/apparent-northern-harrier-in.html
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