I hope someone can give a decent guess at a bird ID from a description. I saw a few each of two similar looking, but different, kinds of bird today; one I think I IDed and one I can't. I couldn't get any photos. Both small, with white underside and grey top side.
The one I think I IDed: White Breasted Nuthatch, looks like a fairly good match for online photos of that bird and a very good match for behavior: When they aren't flying, they tend to be standing or running on the vertical surfaces of trees. They seem to have no preference for which direction they face or run, including standing facing straight down, which I read that most other nuthatches don't. But they tend to avoid the ground and the horizontal surfaces of trees. They are absurdly acrobatic fliers, doing things I used to think only hummingbirds could do. Frequently, one flies fast at an upstairs window, stopping in midair then lifting upward and backward still facing the window, getting above the roof overhang before darting forward again.
So the bird my guess is worse on: Looks nearly the same: Same size and same basic coloring, except the white underside color doesn't extend up the sides of the head (as on the Nuthatch) and the grey upper color is a slight shade browner than on the Nuthatch. The big difference is the unknown bird has yellow/orange legs that are distinctively Sandpiper looking in their proportion to the rest of the bird and the way it stands. But the coloration isn't close to any Sandpiper pictures I found and I don't know whether the size is correct for a Sandpiper. Online, I got the impression a Sandpiper would be a bit bigger, but that wasn't clear. The behavior: walking around the shore of a pond and flying low and level over the pond (and very much NOT flying acrobatically, like the nuthatch) was also indicative of Sandpiper.
Every Sandpiper picture I saw has a top and tail color that was less uniform and less grey. They also mostly had less distinctively yellow/orange legs. So proportionally and behaviorally some Sandpiper, but (size and) colored (except for the legs) much more like the bird that is likely White Breasted Nuthatch.
Any guesses?
BTW, Eastern Massachusetts
Edit: Visited the pond location again today. Someone there guessed those birds were Piping Plover. Looking at online pictures and descriptions of Plovers, I'm pretty sure these birds were smaller than Plovers, and definitely lacked the bulging breast you can see in almost all online Plover photos and the backs were darker grey then Piping Plover pictures.
What time of year would Juvenile Piping Plovers be around half adult length and well under half adult girth? (Or would they be on their own at all at that size). The one online photo of Juvenile Piping Plover I found looked more like these birds than any other bird photo I found, though still not a satisfying match. But that photo had no indication of overall size.
