High δ13C values in red squirrels Sciurus vulgaris explained by a reliance on conifer seeds.

Eurasian red squirrels Sciurus vulgaris have unusually high δ¹³C values compared to other forest rodents, which is seemingly related to the consumption of ¹³C-enriched conifer seeds. To test this hypothesis, we analyzed the hair of wild and zoo-kept red squirrels, seeds of pine and spruce, and feathers of specialized and opportunistic consumers of conifer seeds, crossbills Loxia spp., and woodpeckers Dendrocopos major. Data on the isotopic composition (δ¹³C and δ¹⁵N values) of hair or feathers of other species of forest rodents and birds were obtained from published studies. The range of mean δ¹³C values of hair of wild forest rodents (19 species) exceeded 10 ‰. All squirrel species had high ¹³C content, S. vulgaris having maximum δ¹³C values. In contrast, S. vulgaris kept in captivity had an isotopic composition similar to other captive rodents. The feathers of crossbills were enriched in ¹³C compared to other forest birds (15 species), while seeds of coniferous trees often had higher δ¹³C values compared to seeds of other woody plants. Distinctiveness of the isotopic composition of mammals and birds feeding on the seeds of coniferous trees suggests that this resource can be traced through the entire forest food web.