The rabbit ear mite, or Psoroptes cuniculi, is a common parasite of pet rabbits. Affected rabbits are often seen to be shaking their head, and scratching at their ears, and you may be able to see crusty matter within the ear itself, as well as potentially painful ulceration.
Although common, other problems can cause similar signs in rabbits so ear mites should always be diagnosed by your vet. Any recommended cleaning of the ears when they are infected by ear mites should also be done by your vet rather than at home as it can be very painful.
Ear mites can be transmitted between bunnies, so if you have multiple rabbits together they will likely all need appropriate treatment.