According to the Clemson Cooperative Extension Service, tree squirrels have one to six babies in each litter, and two to four young per litter is most common. The California ground squirrel has as five to eight young per litter.
Four species of tree squirrel live in the United States: the gray squirrel, the red squirrel, the fox squirrel and the flying squirrel. Tree squirrels give birth in the spring to young that are completely helpless and dependent on their mothers. Occasionally a tree squirrel gives birth to a second litter at the end of the summer after her first litter leaves the nest. Ground squirrels breed at different times of year depending on where they live.