Birds nesting in our bathroom exhaust fan vents.
Can you get behind the duct work and vent, such as in an attic space? Otherwise, going up 3 stories is dangerous. Box out with a wire framework from the vent so that vermin cannot get to the louver(s), but air can still flow out. If it is a small vent, I've seen guys glue a piece of metal on the bottom of the flapper, so it is too heavy for birds to fool with, but light enough for air to lift up. A very fine line, I imagine.
Sometimes birds nest deeply into the duct, not just near the exit. You may need to investigate whether or not you have to remove nesting material for duct efficiency.