Bizarro is made possible by millions of years of evolution.
My biggest problem with this cartoon was making the pigeons look huge. It's very difficult to do that if you don't have something of a known size to compare them to. Since this is prehistoric, the only reference I felt I could logically give are trees, but they could also be small, tree-shaped plants.
I consider this cartoon a failure because I missed the obvious answer, which would have immediately both driven the joke home and made it much funnier: that rodent in the bird's mouth should have been a tiny caveman.
Sometimes, I surprise even myself with my dumbassness.