Nearly three weeks since first spotting it, we saw our friendly neighbourhood sparrowhawk again today: in the exact same spot on our fence, and in almost the exact same single-footed pose.
Or is it the same one? This time, the head looks a lot greyer.
But this shot was taken through an open window, rather than through glass, with a different (longer) lens, and in weaker sunlight. Note that the fence looks greyer, too. So I am assuming that all those compositional differences account for the colour difference somehow.