Squirrels come in many different shapes and sizes and with different cultures to match. Grey squirrels are unlikely to invade a human home and are unlikely to fight each other or other squirrels. Brown or red squirrels are more aggressive and black squirrels are the most aggressive by far. They tend to chase and fight other squirrels and during mating season the heat goes way up. Squirrels reproduce on only two days per year, once in the early spring and once in the late summer often going into the early fall. They are pregnant for an average of 30 days but some can last up to 44 days and some as short as 28 days. The real danger starts when they have the babies, they are blind and immobile for the first 4 weeks and then take up to three more months to learn to hunt for food themselves. Once you have a squirrel in your attic you need to act fast and get it out before it gives birth. Black and brown, or as some say, red squirrels are the ones to worry about. It is very unlikely that a grey squirrel will have any interest in your attic and this is the reason why. Gray squirrels are familial, they live together and raise their young together, after all, it takes a village. Brown and black squirrels do not have this familial nature, like raccoons and other predatory species the males hunt the young of other males. They do this to increase the survivability of their own young and to inhibit the procreation of other squirrels.
Why they do this is mostly unknown, it is simply their nature. However, the eventuality of this process is that squirrel females but find a place to hide while they give birth and raise their children and this often happens inside of your attic or home. While they are not going to just barge in through the front door they can climb very well and jump far distances. Common access points include roof vents made of plastic and wall vents which are often made of plastic as well. Squirrels are rodents by species and have the ever-growing incisor teeth that all rodents possess. For this reason, they do not just like to chew, they must chew on everything at all times to dull their teeth down, that or they will grow through the base of their mouth. Squirrels will find an opening they can increase in size and then pass through into small spaces you would not think they would be comfortable in. The attic is spacious enough but going between the ceiling of the first floor seems cramped. They do not feel that way and are comfortable anywhere. This is because their life is in danger, and so are her cubs. Because of this, especially during baby season squirrels become very aggressive and are willing to damage any structure to gain a haven.