We have a major problem with pigeons here in NM. For whatever reason they are protected and thereby cannot be killed. (not that I condone such action). We have an old defunct AC unit on our store roof that is vented directly into the store. Pigeons have been nesting in there and you can hear them in the store, not to mention the mess. we are constantly having to clean debris and feathers that fall through the vent. I brought the issue up with the landlord but he says he can't do anything about it. By the way, I posted the ugly rock. Have you seen it?
I once lived in a place overrun with pigeons. They are a curse. If I were you, I'd wait for winter, when they won't be nesting, and chase the adults out and seal the vent entrance with chicken wire so that they cant get in. Easy and cheap fix if the landlord won't do anything. You could also call in health inspectors, who will force him to do something about it. They do pose a health-hazard