I dont think it matters to them. I have two feeders on an east window one glass and one plastic which get half day sun and at the same time I have glass feeder on my back porch under a slight cover that only gets the afternoon sun. A more important concern would be to not let the nectar turn cloudy from the sun or the hummers wont drink it from spoilage. Especially in hot weather I change mine every 2 or 3 days. The fresher the better , but thats just me. In the same token I dont put a lot in the feeder since I change often, now if you have tons of hummers that changes things.

Right now my feeders are in full sun but as it heats up I will move them over so that there is some shade.

Penny