One thing to keep in mind other than molting and overgrooming is reduced blood flow to the ears. Sometimes loss of fur near the tips of the ears can indicate a heart condition or (if the rabbit is exposed to the elements) frostbite. I have heard of bunnies that had fur loss at the tips of the ears, followed by skin dying at the tips of the ears, and then they were diagnosed with a heart condition. It makes sense in older or heavier rabbits to be sure because heart disease can be a factor. For big bunnies, older means 5+. Big bunnies (both the bigger breeds and overweight bunnies) are notorious for having heart conditions that shorten their lives. Having a wonderful Californian boy who's just turned 1yr old, I am a bit sensitive to the knowledge that he may not live as long as our small bunnies. For medium and small bunnies, older means 7+.