my rabbit is losing hair on his ear .

Question
I was just bringing my rabbit in from the backyard and i've noticed that his left ear  on one side has a bald spot on it.

Answer
It could be fur mites, ear mites (especially if it is near the ear opening), the start of shedding his fur in that area, a patch he rubbed off, or where a wild animal took a chunk of fur off. Without any other clues as to what is causing it, I really cannot narrow it down. If there are other bald patches anywhere (especially near the face and neck) and/or if he is scratching, it is probably either fur mites or ear mites. In which case, a vet can get you medication to get rid of them.