Red and Grey squirrels are solitary for the majority of their lives, maintaining distinct, but frequently overlapping (up to 90% in some cases), territories and chasing off other animals that invade their interpersonal space; a ‘zone’ that varies in size from squirrel to squirrel. Studies on both Reds and Greys suggest that there is a dominance hierarchy among neighbouring territory holders, with dominant individuals holding higher-quality habitats than subordinates, but direct conflict is rare.
There are some reports of communal nesting in both species, but such incidents are rare and typically occur during particularly cold weather in the winter and spring. Jessica Holm observed that Red squirrels sharing a drey appeared to be familiar with one another, suggesting a familial connection, but this is speculation. In one instance of Greys sharing a nest 20 individuals were found in a single cavity.
Work by John Koprowski (more in Dispersal & Aggression/Dominance) at the University of Kansas found that Grey squirrels commonly nested in groups of two to nine individuals and they nest-shared throughout the year, although most cases were during the autumn and winter. Koprowski observed 245 communal nesting events in his tagged population and found single-sex groups to be the most common occurrence, with both all-male and all-female groups occurring, and there was a strong kinship influence – 43 (83%) of the 52 nesting groups containing more than one adult female were made up of related females. Koprowski observed only nine nesting assemblages between unrelated adult females.
In most cased I have come across, the squirrels share dreys with members of the same species, although work by Ralph Ackerman and Peter Weigl at Duke University in North Carolina, observed American Reds (Tamiasiurus hudsonicus) and Greys sharing the same nest boxes. Simultaneous sharing of nest boxes has not, to my knowledge, been observed between Eurasian Reds and Greys, although ‘timeshares’—where Greys will use Red squirrel nests when the Reds aren’t around and vice versa—have been recorded and is a proposed method for squirrelpox transmission.