There are several species of squirrel, but tree squirrels are the the most common nuisance for homeowners because they inhabit attics and cause damage. Squirrels are typically discovered because they make a lot of noise in the area they inhabit. Squirrels are abundant in the wild because their numbers far exceed the number of predators. The large population and decreasing natural habitat due to construction in many areas causes them to seek shelter in houses. The cool months of fall and winter cause squirrels to seek the warmth that is found in attics.
Removal of any diet source should be done: bird seed for example, because they attract the squirrels is the first order of business. Gutter guards and covering down spouts will reduce the amount of squirrels entering through the roof top and fascia boards. There are no poison baits on the market that squirrels will eat and die. There is none that are labeled as squirrel bait, just mice and rat bait. Live trapping squirrels is the only way to remove unwanted squirrels.
The trap should be located along the path of entry and places of known foraging activity. Sometimes a professional trapper with a nuissance control license is needed to not only capture the squirrel, but relocate it as well. Trapping a squirrel is really the only way to get rid of it in your attic, once it has chewed a hole.
It is important to seal up and repair all entry points after trapping the squirrels so they no longer have access.