I have a beautiful glass globe hummingbird feeder that has a rubber stopper. How can I stop the nectar from leaking out of the stopper?

If there is debris on the stopper it might be preventing a good seal. Try removing the rubber stopper and wipe it off to remove any debris that might be on it. Clean out the hole that the stopper goes into as well. This will help ensure a clean seal between the hole and the stopper. Put the stopper back into the hole, making sure it is a tight fit. If it still leaks you may need to buy a new stopper.