Photo by Happy Snapper
Dusk is a melanistic barn owl, she is the opposite of an albino. This rare gene mutation occurs 1 in 100,000 with none surviving in the wild. Dusk was hatched in a falconry centre in Lincolnshire and now lives at the owl sanctuary at Winterton, Norfolk..
That's my initial thought. The leukistic owls, where they end up all white, also do very poorly and are usually blind to some extent. It usually seems there is more than just the generic switch for color that gets flipped the wrong way.