The reasons why you may have flying squirrels in your attic is that the top of your home at the ridge vents, the gable vents, or the soffit vents have been chewed through as it is very easy for flying squirrels to do. They can jump from a tree as far as 175 feet away and land on the smallest area on your home and then climb in or chew their way through or in some cases will slip inside the attic behind the gutters. For all these reasons, flying squirrels removal should be done as early as possible and with correct measures.
The only way for flying squirrel removal is to make sure that all areas on the top of the home are physically searched and sealed correctly. This means no foam in the can (which is not UV rated and easily chewed) or caulk can be used as these weak materials will not keep flying squirrels out for very long, if at all.
The only materials that will keep flying squirrels out would be flashing which is metal as well as custom wire cloth which is why Thoroughspect uses 18 gage 1/4 inch spaced hardware cloth so they simply cannot chew through it as flying squirrels are voracious chewers and fit through very small spaces.