I have to say, I am not convinced that this is a female Blue-fronted Redstart. The tail pattern does not look right; it should be really well defined and should also be apparent on the underside of the tail - which it is not; the upperparts look too grey; the pale tips to the greater coverts are thin and off-white (on photos of first-winter Blue-fronted they appear broad and very pale); the under tail coverts are rather bright.
The tail pattern should be stunningly obvious in the field which it does not seem to have been as it is not mentioned.
Finally, the usual call of Blue-fronted Redstart is a really distinct dry rattle; very different to the Robin-like call described.
In all in think this is 'only' a Black Redstart!
BTW Blue-fronted is already in the HK list (Cat E) on the basis of one in Pok Fu Lam CP on 25 May 1997.
