[cypher]

It's hard to say what might be causing that, sometimes they can get bald patches from trying to squeeze through holes or gaps in things that aren't quite big enough, this doesn't really look like that but it's always worth giving the cage a check to be sure nothing in there could be causing a problem.
Keep an eye on it & if it continues to get worse then it may be worth having your vet check it out.
In the meantime you could try giving some flax seeds & a tiny drop of evening primrose oil daily, I've found this to be very effective for hair loss if there's no other underlying cause that needs to be treated.