Usually when a hummingbird feeder is full when night time comes around, and empty by morning, one has to wonder what animal was in the hummingbird feeder at night? The answer is usually bats.
Bats have different ways of liberating the hummingbird nectar from the feeders. Some bats will land on a feeder and drink the nectar directly out of it. Other bats will fly around in circles and use their long tongues to reach in and get a sip of hummingbird nectar with each pass.
There are two schools of thought on bats in a hummingbird feeder. The first one is that bats while wonderful creatures that do a great job of eating all sorts' of bugs; however, they also carry rabies and other diseases. The other school of thought is that bats do a great job of eliminating mosquito's, which carry West Nile Virus and they are just as fun to watch at night as hummingbirds are during the day.
If you are one that subscribes to the first school thought, that bats carry disease and should not be stealing the hummingbird's nectar, the only things that can really be done is prevention.
If at all possible, make sure your home is secure to prevent bats from nesting inside your attic or walls. Look for any gaps, cracks, or holes in the outside walls and eaves of your house where a bat might squeeze in. If you see any small grease like stains near a hole or crack, this might be an indication that a bat has, or has tried, to squeeze into this location. Seal up these holes and cracks. Or put mesh screening over the opening if the hole is a vent and suppose to be there. This might help keep the bat population in your area a little lower.
To get the bat to stay away from the hummingbird feeders, the only thing to really do is to eliminate potential access to the hummingbird feeders at night. This can be done by taking the hummingbird feeder in at night or covering it up so that bats cannot get to it. However, the feeder must be replaced or uncovered every day before dawn as hummingbirds need the nectar as soon as they wake up. Hummingbird will need 25% of their daily intake of food and nectar as soon as they wake up in the morning (which is usually just before dawn).
If this is just too much to do, you can do what we do and decide that it is ok to simply coexist with the bats. It is great to have the bats eat as many of those nasty night-time bugs as possible. Plus they are kind of cool to watch.
A bat caught in the act by Dr. Raul Erazo, MD of Colombia. 6/11/2012