I have encountered many critters that make their home inside dryer vents. Rats,Squirrels,Opossum,Rabbits & Snakes are among my findings. If your dryer vent is accessible to these critter they may occupy your vent and nest.
Pest intrusion into exhaust systems can lead to health hazards and worse. If birds or squirrels have the chance, they will build nests in dryer vent systems and create a very real fire hazard. Airflow restriction increases drying times, wasting energy and shortening appliance and clothes life spans. This restriction also blows lint back into the heating element making a recipe for a fire.