Help! What tree is this? A poplar!

This is a tall, not particularly straight, grey-barked tree which is on the edge of the woods at my father's place in East Tennessee. It's a fast-growing tree, thin, and if I remember from this summer the leaves are longish and rough around the edges. None of the on-line tree guides show anything like this fruit, though. Here's a cardinal eating a seed; he'll drop the long, paper-thin samara to the ground, which is littered with them. Also, an eastern phoebe sitting between two of the flower-like pods, which may help with size. Then there's a picture of the whole tree (the front one), the crown, and a close-up of the bark.

Does anybody have any idea what this might be? I'll have to make a point of looking at it this summer, but at the moment there are absolutely no leaves on it, and my father doesn't recall the flowers being prominent in any way.

Thanks, Rainyj317! Tulop poplar, aka yellow poplar. The Virginia Tech dendrology site says "base whorls of samaras persist on fruit into following spring and resemble wooden flowers high in the tree" which is exactly right, but which I couldn't find a good picture of; even theirs isn't late-stage like the ones below.