These diurnal rodents adjust easily to new types of environments, although they are especially common in woodlands and forests. They also frequently make appearances in places with high human presences -- think urban parks and yards of residences. Grey squirrels generally den within trees, but also build nests out of foliage in the absence of any suitable hollows. Their population is high and increasing, notes the IUCN Red List of Threatened Species' 2008 analysis. As a result of those strengths, they are "least concern" animals with regard to possible endangerment. They bear litters one or two times annually.