Originally Posted by
Quickbeam
It's almost certainly the remains of a hawk's dinner. I forget which, but one species of hawk (Cooper's or Sharp-Shinned maybe?) eats the bird's head and leaves the rest. Anyway, headless birds are the calling card of one particular hawk species.
How much sustenance can you get from a pigeon head? It's obviously meant to be a warning to the rest of the pigeon population.