Some of the most unusual visitors to Tucson, Arizona hummingbird feeders are two species of nectivorous (nectar-feeding) bats, the endangered Lesser Long-nosed Bat (Leptonycteris curasoae yerbabuenae) and threatened Mexican Long-tongued Bat (Choeronycteris mexicana). In the late summer and early fall here, large numbers of these bats will visit hummingbird feeders at night, sometimes quite late in the evening, and they can quickly drain every feeder in the area. These hungry bats often drink so much sugar water that their stomachs bulge as if they were pregnant. The QuickTime movie below shows bats visiting one of my hummingbird feeders. The video is at actual speed, these bats really do feed this amazingly quickly (notice the bubbles going up as the sugar water is drained).
You are most likely to have nectivorous bats at your feeders if you live in an area of natural desert vegetation on the outskirts of Tucson. Other areas here in southeastern Arizona can also have these nectivorous bats show up during fall migration. In my hummingbird feeders, I just use a mixture of 4 parts water to one part white sugar, and both the bats and the hummingbirds like it.