Yes it is a Robin but I’ve never seen that before. I don’t think that’s where the eggs are. I’m wondering if it’s more of a growth of some type. Maybe someone else will know more.

That thought actually crossed my mind. Here is another shot where it is more evident.

That's really weird. I've tried to find an answer on Google, but haven't gotten one. Is the robin acting normal otherwise? I don't really know the anatomy of birds that well, if this is even possible, but when I had chickens, sometimes an egg would get loose from the oviduct and end up in their abdomens. But I don't think the base of the tail is really open to the abdomen. Maybe it's just a growth. I just don't know. I wonder if you have someone around your area who is an ornithologist and you could ask them and show them the picture?

I started to think about that too, but not much thought yet. I am in suburban Chicago. Maybe someone at the Brookfield Zoo. If I come up with something I'll report back.