Birds love dryer vents for a couple of reasons. It’s great protection from predators, and the warm air is good for incubating their eggs and keeping them warm in cold weather. To prevent birds from nesting in your dryer, there are two things you can do:
First, make sure the flapper on your exterior vent is not held open by excess lint buildup. If the flapper does not close tight, a bird will open the flap with its beak and head inside to nest.
Second, install a screen on the outside flap. Make sure the vent is clean first and the screen you decide on has large enough holes for lint to escape, but still keep birds out. Once it’s installed you have to keep an eye on it. Check the screen every couple of months for any buildup of lint. Even if the holes in the screen are large, lint is still likely to accumulate.