Wildlife, such as birds and mice, can enter homes through dryer vents. However, securing this entry point requires great care. Secure dryer vents improperly and you have created conditions to start a fire. Say you use 1/4-inch hardware cloth to secure the opening. That screen reduces the air flow and allows lint to collect on the screen. As the dryer heats up it can ignite the lint and voila, you have a fire. According to Consumer Reports (Jan 2011 p. 3), lint filled dryer ducts cause 4,500 fires a year in the U.S. alone.
Before securing your dryer vent, be sure to consult local and state codes. My understanding is that some states forbid any screening of dryer vents. In these cases, the best you can do is to be sure that you are using an aluminum dryer vent hose and not the vinyl to at least keep the mice from entering the home.
For those locations where you can secure your dryer vents, then consider this device. It uses a ball that is pushed out of the way as the dryer runs then gravity drops back down when the dryer stops. No screens but still prevents animal entry.
Finally, be sure to monitor the device to ensure it continues to work properly. Daily following the install then weekly or monthly as your findings dictate. Of course, don’t forget to have it cleaned regularly.