Bluebirds and a baby robin

Eastern Bluebird female, Charles River Peninsula, Needham, MA

I heard a commotion of bluebird songs from all over the CRP this morning. Could there be new bluebird fledglings?
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.

There was no sign of bluebird fledglings (though they might have been hidden). So why all the fuss over a baby robin? My first impulse was to think that the bluebirds were being protective. As I approached the chick and the chick began to voice distress the bluebirds became more upset. (In fact, I don't think I've ever seen a mother bluebird in full scold mode before). Other birds flew in to see what was happening (in one beautiful scene--unphotographed--the female bluebird and a female orchard oriole perched next to each other on a wire).  Were the birds gathering together to defend this helpless little thing? (By the way, no sign of a parent robin anywhere).

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.