Northern Harrier (Marsh Hawk) Circus hudsonius
This North American counterpart of our Hen Harrier Circus cyaneus has until recently been treated as a race of Hen Harrier. Some authorities have recognised the two taxa as distinct species for a while, others still consider then as a single species but the BOU adopted the split in 2015.
Separation from European Hen Harriers is extremely difficult and, for juveniles at least, it seems that any of the characteristics that are indicative of Northern Harrier can sometimes appear on Hen Harrier too. It would, however, be extremely unlikely for a Hen Harrier to show ALL the features of Northern Harrier, and on this basis a small number of Northern Harriers are believed to have occurred in the UK and Ireland. One of these was in Norfolk in 2010 and is shown below, along with some poor shots of birds within their normal range. Note how the intensity of orange varies according to the light - the same is true of juvenile Hen Harriers.