This one has been driving me nuts. I'm reasonably sure (for a novice) that it's some sort of sandpiper or the like but, none of the pictures I have found seem to match well enough.
Saw this one at Grimes Glen in Naples, NY on 27May2018 at 11:40am (county of Ontario). This is a small forested stream with several waterfalls along it's length in a sizeable ravine.
Here's the video: https://imgur.com/a/Y0Ho2i8
I realize its not great, looked better on my phone. Wings did not appear spotted but rather a uniformly dark color. Back a lighter grey with a white underside; beak was not dark; breast mottled gray?