Hummingbirds often hunt insects in trees, also. I've got one tree that blooms sometimes, small greenish flowers. Nothing pretty, but they really do attract a lot of insects when blooming. The hummers like to hunt the bugs on that one and also on live oaks that are large enough. BTW, winter hummingbirds can be given nectar that is a bit sweeter than at other times of year. I guess that compensates somewhat for lack of their usual amount of flower nectar.
Oh...BTW, I just spotted a hummer hunting in a tree just outside my window! I'm so glad they're getting fairly warm sunny days this week...that'll give them a chance to catch up on their "protein"!