My camera was struggling in the low light to focus on the robin that was impaled on this metal fencepost located just down the street from where I live, so apologies for the slightly fuzzy image. I'm sure most people who passed this grisly sight assumed there is some young budding sociopath living around us, but this was most likely the work of a shrike.
Shrikes are relatively small birds of prey that impale their victims on spikes, thorns, barbed wire, and, apparently, thin metallic fenceposts. They do this for storage – they don't need all that meat all at once, being fairly small, so it makes sense that they would want to keep some food around and save energy instead of continually hunt.
This particular fence is located outside of a seminary with very large pines on its grounds. You can regularly see a variety of birds coming and going from them.
The above may also have been the accidental work of a hawk.
Some months ago I was walking past this very spot and a hawk swooped down not two feet from me, grabbing a mouse that I obviously didn't notice. With the mouse still twitching in its claws, I remember the hawk looking up at me, seemingly saying, "Get the fuck away from my mouse."
I left it and the mouse alone. Sorry little buddy. No, I do not want it. Please. Keep it.
But why might this have also been the accidental work of that hawk? The hawk (in this case, I think it was a red-tailed hawk) is a much larger bird of prey, bigger than both shrikes and robins. The robin from this photo may have simply been perching on the fence when the hawk swooped down, and its weight could have impaled it on the spot. The only thing that makes this seem unlikely is that I would think the hawk would have simply eaten the robin right then and there, instead of letting it stay impaled.
Anyway... Nature is pretty metal.
Update: I am currently listening to a northern shrike calling outside of my window as I type this, so... mystery solved.