Question for a Master Naturalist

I went to visit a friend of mine in Elk River. He asked me a question noting that I was a master naturalist and might know about this. When you are a master naturalist, people tend to ask you questions. I don't claim to know much but I am interested and will share what little I do know. A naturalist isn't someone who knows everything but someone who knows more every day. His problem was with his hummingbird feeder. At night the feeder is full of nectar and in the morning it's empty. Something is sucking it dry at night but not breaking the feeder. I asked about raccoons. He said he had called the DNR and they said it probably wasn't a raccoon because a raccoon would most likely damage the feeder. He asked about bats. The six species of bats in Minnesota are all insectivorous. They eat mosquitoes, moths and beetles by catching them in their cupped tails and transferring the food to their mouths while in flight-no easy task I would say. If it's not raccoons and not bats, there can be only a couple other options. 1. His teen aged kids are jerking his chain or 2. A rare giant nocturnal hummingbird.