|Eastern Bluebird female, Charles River Peninsula, Needham, MA|
I saw a small shape on the freshly mowed path. Aha! I thought. A closer look revealed a baby robin, fully feathered but unable to fly. This was the focus of bluebird attention.
Here's a video version
My other thought was that this was just a bluebird freak-out--an overwrought response to an unfamiliar being making noise close to their nesting box. This doesn't explain why they got more upset as I approached, though maybe they were simply responding to the intensified robin cheeps. The red-winged blackbirds, for example, are currently freaking out over anything that happens to get close to parts of the fields where their babies are hiding. (One was giving a male bluebird a particularly hard time).
By the way, here's why I didn't intervene to help the chick myself.