This fine fellow was singing on a tree by Green Lake.
He's a male tree swallow, calling to advertise that he's staked out a hole in a tree suitable for nesting.
And it looks like his advertising has paid off.
After a while, the female flew away, with the male in pursuit.
Tree swallow courtship is a complex affair, so it's probably too early to predict whether there'll be babies in that cavity.
But I'm going to keep an eye on that tree.