From the way she was behaving I figured she must have a nest in the weeds along that long side. Then I saw it, barely:
As you'll notice, it wasn't in the weeds. She had built her nest in the arena! It was just over halfway down and about 15 feet off the rail. I happened to glance down because I thought I saw a rock. (the ground where the ponies live is very stony. We constantly pick rocks from the arena.)
Nope it wasn't a rock. It was four eggs in a nest made of pebbles and dirt with a few little twigs! (not quite my idea of comfort! lol)
Next time I rode I set up some cones so I wouldn't trample the unhatched babies. Momma bird (daddy bird?) didn't get as upset this time. Probably because I made sure to give the nest area a wide berth.
I couldn't get a clear pick of Parent Bird. But I did some research online and I am fairly sure it was some sort of plover or piper. The one that matches closest to my non-professional observations is a piping plover.