That pierced egg on the ground was surely work of a house wren. Last year I had 4 eggs in a box which I was not aware of, and by the time I was able to get another wren guard up, a wren had pierced & thrown out 3 of the 4 eggs on the ground. That lone egg was incubated and did fledge. In regard to the robin, it couldn’t get in & probably would not try, since they are not cavity nesters. They are curious like all other birds and will sit atop of anything for a while. Robins & bluebirds are both members of the Thrush species of birds, like cousins, and do cohabit quite well usually. Good luck – keep us informed.
