At Peebles Island, a few flowers and leaves against a backdrop of gray

At Peebles Island, one tree flowers while those around it are bare.

Note the insect in the middle picture.

What could this tree be? I think I'll look in the cherries and plums section of my book.  It does seem to resemble American plum.

And here we have another kind of tree which has nice catkins.

I looked on the internet at pictures of catkins, but I didn't see any that matched this. A lot of catkins in The Sibley Guide to Trees seem to be more yellow and fuzzy.  Could it be that these will turn more yellow and fuzzy when spring is a little farther along? Here's what the trunk of the catkin tree looks like:

Also amid the gray leaflessness, there are some green leaves in the understory.

Here's a closer look at the source of the green leaves. I'm thinking common buckthorn because that's what was in the understory here in summer, but other than that, this doesn't especially look like buckthorn. Is it another understory tree that is less noticeable in summer?

Closer to the shore, the boxelders and the weeping willow are showing leaves.

And the red maples are flowering.