Context. Nest or roost sites are important for social thermoregulators – not only because the locations provide shelter from harsh climates, but also because they provide sites for social aggregations. Nest use can therefore be informative about selection pressures facing social thermoregulators.

Aims. The aim of this study was to assess seasonal changes in nest use of sympatric northern (Glaucomys sabrinus) and southern (Glaucomys volans) flying squirrels. Local sympatry at our study site allowed us to evaluate nest use by individuals of both species subject to similar nest availability. We hypothesised that southern flying squirrels should be more selective than northern flying squirrels, especially in winter due to lower cold tolerance by the southern species.

Methods.We used radio telemetry to track 57 squirrels during 2019–2022 at a site in central Ontario, Canada. Each squirrel was tracked during the day to their nest site, and tree characteristics – including diameter at breast height, tree species, nest type and decay class – were recorded.

Key results. Northern flying squirrels used both coniferous and deciduous trees, as well as a mix of cavities, dreys and subterranean nests. Southern flying squirrels nested most often in deciduous tree cavities and used dreys less frequently than northern flying squirrels. The only significant effects in regression models, however, were effects of tree diameter. Both species used large-diameter trees in the winter and summer, and these effects were larger in the winter months. In both seasons, southern flying squirrels used larger trees than northern flying squirrels.

Conclusions. Our study results were consistent with the hypothesis that nest selection is associated with temperature and squirrel aggregation size. Both northern and southern flying squirrels used large trees during summer and winter months, as would be an expected requirement for aggregation; however, this effect was amplified in southern flying squirrels and in the winter.

Implications. Cold ambient temperature is an underlying factor in winter months, creating the need for social thermoregulation and increased squirrel aggregation sizes, especially in the small-bodied southern flying squirrel. This in turn leads to a need for large-diameter cavity trees for nest groups to occupy during winter.