That is exactly what happened with our nest of little House Finches that we had on our patio a few years ago. One day I heard a lot of screeching and carrying on outside and saw a scrub jay sitting on the nest pecking at the baby birds until they fell out of the nest. Three were already dead on the ground. I chased him away, but the jay eventually killed all but one of the babies. I could see no reason for this at all. I was so upset, I felt responsible for these little birds, but I couldn't protect them.
The interesting thing was, there were several Finches nearby pitching a fit over what was happening. One of them surely was the mother bird, but the others were behaving like a family and were just as alarmed over the babies being attacked.
I guess it is natures way of keeping the bird populations balanced out, the jays are just doing what jays do. But, "survival of the fittest" is hard to watch. I think most people's first instinct is to try to interfere and protect the innocent weak ones. (I know, it is this concept in government that the liberals have taken to extreme.)