Sometimes the flaps on your dryer vent get dirty and don't close properly. This allows all kinds of dirt and debris to get inside the dryer ductwork, including things brought in by vermin seeking a nice warm home. That appears to be the case here, in this Lewes, DE home. As you can see in the picture, in addition to the lint left behind by the dryer exhaust there are seeds and grass, which indicate mice, rats, or birds have been hanging out in this nice warm shelter.
Lawson Home Service's technician cleaned out the flex-ductwork and flap to improve the amount of airflow and ensure the unwelcome visitors did not return in the future to wreak havoc upon this unsuspecting homeowner.