In recent years I've found a few Black Brants in Norfolk and also a few birds that haven't quite been right, which I've identified as hybrids with Dark-bellied Brent Goose.
Those that I've identified as hybrids have been rather variable but typically show a less contrasting white flank patch than Black Brant and/or a smaller white neck patch than Black Brant (often incomplete at the front) and/or a darkness to the back and belly that is intermediate between Black Brant's and Dark-bellied's. A few have been closer in appearance to either form and have left me in some doubt as to whether they are hybrids or not.
This winter I have seen at least three birds that have shown a full Black Brant-like neck collar, prominent white flanks and fairly dark back/belly, but seem to have shown a consistently greater contrast between the black neck and dark back/belly than I would expect on a Black Brant. I am aware that pure Black Brant can vary considerably according to the light, sometimes showing absolutely no difference between the solid black of the neck and the solid black of the back/belly, and in other lights showing considerable difference. The birds I'm talking about vary according to light too, but seem to show more contrast more of the time than I would expect on a pure Black Brant. Two of them appear on my webite - http://www.gobirding.eu/Photos/BlackBrant.php: they are the birds at Wells on 7-Dec-08 and Burnham Overy Marsh on 31-Jan-09.
The Wells bird has a distinctive pale mark in the black of the upper belly, right hand side - it is the same bird that is captioned as a Black Brant in the British Birds magazine that arrived this morning. What do others think - are these pure Black Brants or hybrids?
I've not seen Black Brants in their native range and photos I've seen (mostly from the North American population) reflect the same range of variation in contrast according to light conditions that I'm used to seeing in what I have identified as pure birds here.