This is a photo of the dryer vents on the sides of our buildings. The upstairs units will have vents that go through the roof. These should all be covered by grates to prevent birds from nesting and entry by rodents. They do occasionally blow off or fall out.  

If you find that your dryer isn't drying in a timely fashion, perhaps it is time to have the vents cleaned by a professional. Having regular maintenance to your dryer vents, both inside and out, will insure that the covers are present and the ventilation system has been propertly connected in the attic.  

A thorough inspection should include the roof vents.