Nectar Guards
Nectar Guards are thin plastic membranes which are placed over the bottom of the feeding ports on an Aspects HummZinger Ultra feeder. They have slits in them so the hummingbird's beak can penetrate the membrane and drink the nectar, but ants an bees cannot.
Here is a
photo of the Nectar Guards as they come with the feeder:

The illustration shows them still on their carrier. You break
them off of the carrier and press them onto the nubbin underneath the feeding
port.
And here is a close-up showing the slits.

Finally, here is a fuzzy illustration of the Nectar Guard in use. (It's the pink cap on the bottom of the illustration.):

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