I would first wonder are they black vultures, or turkey vultures? The black vulture is a bit of a predator. Were the birds hanging out on your deck railing black vultures, then I would wonder was there something beyond your deck the birds might think of as a potential meal. Black vultures MOSTLY are scavengers, but they will totally kill small, typically newborn animals, and they will totally eat eggs of any animal they can. If your deck railing birds are turkey vultures, then I'm unsure of what might attract them to your deck, outside of your obviously fantastic deck railing building facility.
You just can't legally hurt those birds. I can imagine that were you to own a nice adult cat, then that cat, being unconstrained by human jurisprudence, could do whatever it wished with the birds.