What would you suggest to eliminate and discourage birds from nesting in our exhaust fan vents? The birds lift up the flaps on the exhaust outlets and enter the duct work. The vents are about three stories up from the ground.
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.