As far as large pests in your attic are concerned there are two main threats: birds and rodents. Birds usually build their nests high up and in an enclosure like the space between splitting tree branches. But, a nice opening in an attic is attractive to birds as well. This is why it is important to make sure any ventilation holes are screened and any damage is repaired.
As far as rodents go, they are looking for nesting quarters as well. Sometimes they are just looking for food. Regardless their reason for being in your attic, the end result is never good. Rodents are known to carry diseases that are harmful to humans. They can pass this disease on by leaving behind feces and saliva.
Besides contaminating your attic with their excretions, rodents have been known to chew on wires, insulation and items that you have stored up there. It is a no-win situation when it comes to rodents in the attic.
Any gaps leading into the attic must be sealed. Rodents can get into tiny cracks as small as a quarter inch-thick. If you already have a rodent problem, it can only be dealt with after the attic is properly sealed. This will prevent the infestation from occurring again.
Baits and traps can work if applied correctly. If you have the time to set and monitor them and then clean up afterward, this project may be for you. Many people choose to hire a pest professional to get the job done correctly, right from the start.
After your pest problem has been eliminated it may be necessary to repair insulation and other items in the attic. Getting a full insulation replacement may not be a bad idea if you have an older house with older insulation. The money saved from installing the newest insulation will probably pay for itself in as little as 6 months to 3 years time