My coworker was telling me that he has a Corkscrew (sometimes called Curly) Willow tree in his yard that has bees all over it, as in covering. I am assuming that this time of year they are pulling propolis? The tree is definitely a spring blooming tree.
(Note: not a swarm, from his description, they are gathering something. My guess was propolis, but the sheer number he was talking about sounded large. - Thousands)