Red squirrels build nests, called dreys in the forks of branches, close to the main trunk. The drey consists of a hollow ball of twigs and leaves, which is then lined with soft hair (alpaca fibre in Fowberry squirrels’ case) grass and moss. Summer dreys may also be constructed, which are flat, less protective structures used for resting during daylight hours.
As an alternative to dreys, some squirrels may use natural holes in trees, which are known as dens. Often two or three dreys are in use at any one time; these may be close together or wide apart, depending on the squirrels’ range. In the winter and very early spring squirrels of all ages and both males and females may share dreys but only if their territories overlap and they feed close together or know each other. Drey sharing usually stops in late spring and summer when the females are raising their young.
The ‘chuk chuk’ noise a red squirrel makes is a vocalisation used with tail waving and foot tapping when it is agitated about something – like another squirrel hogging the nut box!
The mating season often starts on warm days in January with the squirrels chasing each other through the branches or around a tree trunk.
The female red squirrel may produce two litters in a good year (45 – 48 days after mating), one in the spring 
(April) and the other in summer (August). There can be up to six young born, more usually two to three babies (kittens) in a litter. The breeding drey is usually a little larger than normal with a thick, soft, lining of grass and hair. If the mother is disturbed, she will carry her babies in her mouth, one by one, to another nest, which is sometimes quite a distance away.
The young are born blind and naked. As they develop, the female spends more and more time away from the drey, and by the time they are three weeks old she may leave them for hours at a time.
At seven weeks the young begin to venture away from the nest and at nine to twelve weeks they are weaned and become independent. Their fluffy, darker baby coats change into the adult colour.
The young squirrels’ survival is determined largely by their first winter, with up to 85% of young, perishing during harsh conditions. Only females bring up the young and are territorial over their brood, with the male taking no part in the rearing of his young.
The success of the breeding season and raising of babies, depends on the seed crop of the main trees where they live. Where there is plenty of food, squirrels build up body fat and many survive the winter in good condition. This means they will start breeding early the next year and rear many babies.
In a year when there is a shortage of food the squirrels do not put on much fat and they may die from starvation or disease during the winter. Most of the survivors are not fit enough to breed successfully.
Grey squirrels have evolved with better digestive systems to cope with the poisonous tannins in acorns.