When I was at 30th Street Station, outside, waiting for the SEPTA train, I noticed that all the steel girders had these weird little strips of spikes all along them. They were like little stalagmites sticking up. It took me a couple of seconds to realize that they were put there to keep the pigeons off the griders. Sure enough, on the few girders without spike strips, several pigeons were roosting.
That’s a lot better solution than the recorded cannon boom they play at the Park City Mall to scare birds away. That freaks you out if you don’t know what it is.