It wouldn't evaporate in just one day. If you change the nectar every 3 or 4 days you shouldn't even notice an evaporation, so you may have a leak, or something is helping those Hummers drink it. Just one Hummer couldn't drink that much. Do you have Oriole's there, or if it disappears overnight, it could be a bat.

The water could evaporate making the sugar content too high, but it should be consumed or replaced before that occurs. I only put a few ounces in my feeder at a time and check/add a couple times a day. Then again I only have one feeder and a few birds.

Ive only seen one hummer late afternoon, but I dont watch the feeder all day. I also dont put alot of nectar in the feeder...I change it every 2 days its getting hot here in minnesota. thanks for your comment