Winter nest trees of sympatric northern (Glaucomys sabrinus) and southern (Glaucomys volans) flying squirrels: a test of reinforcement in a hybrid zone

Abstract

Shifting range boundaries can lead to secondary contact of closely related species, which might in turn lead to hybridization when the evolution of reproductive isolation is incomplete. We examined winter nest use of northern (Glaucomys sabrinus (Shaw, 1801)) and southern (Glaucomys volans (Linnaeus, 1758)) flying squirrels in an area of recent secondary contact and known hybridization in Ontario, Canada, to test for evidence of reinforcement due to different and diverging nesting behaviours. We radio-collared 26 flying squirrels (12 G. sabrinus and 14 G. volans) between two survey periods (winters of 2008–2009 and 2019–2020) and identified all nest trees used by individuals throughout each winter. For each nest tree, we identified the nest type and collected tree classification information to compare differences in nest use between species. We also present a novel application of habitat suitability modelling to test for evidence of divergence in nest use through time, which would suggest reinforcement. We found southern flying squirrels used a higher proportion of cavities in large, hardwood trees, whereas northern flying squirrels used more external nests and softwood trees. Conditional probabilities provided some evidence for increased differentiation in nest use by flying squirrels through time. Overall, we found relatively little overlap in winter nest use between flying squirrel species, despite evidence for hybridization at this site, and some weak evidence for increased divergence between species in nest use over 11 years.