White-winged black tern at High Rid Reservoir

At lunch time I called in at High Rid reservoir near Horwich for a look at the juvenile black tern that has been present for a couple of days, only to discover that it was actually a white-winged black tern! As soon as I saw the bird I noticed the white rump and dark saddle, but on closer inspection it also showed what appeared to be a shadow of a breast patch, though this wasn't always visible. A breast patch is usually regarded as a diagnostic feature of juvenile black tern which probably explains the misidentification for the first two days of its stay, but apparantly white-winged black tern can also occasionally show this feature. I suppose a hybrid is also a possibility, but I have no idea how common they are. Probably a lot rarer than white-winged black tern! This is most likely the same bird which was at Rishton Reservoir on 4th September, which also had a faint breast patch.