The perennial question, where do dead birds go?, has been answered on at least 4 occasions on this blog*. The plain, and the only, answer is that they fall down from the sky and then get eaten.
This dead robin was on a sidewalk near my house a few days ago. There were flies and dark gray beetles on it. I don't know what those beetles were. And I couldn't get a good picture of them, because all I had with me was my iPhone. The thing over the bird's eye was one of the beetles. In the next picture you will see that it moved between the 2 shots.
So once again, dead birds get eaten like everything else that is dead.
*See this and this and this and this post.