I thought it worth posting regarding the id of this bird. Whilst there is general agreement that this is indeed a female Black Redstart no-one has really concluded why!
In actual fact, this is quite a straightforward identification in a complex sort of way!
Although this varies depending upon the light conditions the bird is clearly quite dark below, with the breast and throat being a uniform dark brownish-grey and with quite extensive dark reddish brown on the undertail coverts extending up onto the centre of the belly. This can only fit a female Black Redstart of the subspecies rufiventris (which breeds widely across the Tibetan Plateau) or xerophilus (from the Kunlun Shan). Xerophilus has quite a small range and is possibly best treated as a synonym of rufiventris. Confusion between Black Redstart and Hodgson's redstart is only really possible when dealing with females of the Central Asian taxon phoenicuroides which compared to rufiventris is a distinctly paler below and a purer grey on the throat and breast. Separation of female phoenicuroides from female Hodgson’s would be on the basis of the rufous being restricted to the undertail coverts of Hodgson’s and also in it often showing a whitish centre to the belly.
It is perhaps not surprising that the only previous record, also in spring, was a male rufiventris. Pheonicuroides is possible in Hong Kong, but probably more likely in autumn than in spring (based on dates of other Central Asian vagrants to HK); identifying HK records of Black Redstart to taxon is worthwhile if possible as it seems likely that the complex will be split at some point and rufiventris and phoenicuroides are possibly valid species.
