Difference on cone size preferences between two coniferous species by Great Spotted Woodpecker (Dendrocopos major)

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.

Great spotted woodpecker (Dendrocopos major) and red squirrel (Sciurus vulgaris) prefer different cone features of European larch (Larix decidua)

Specialized conifers-seed-eating animals, such as crossbills (Loxia spp.), woodpeckers (Picidae), squirrels (Sciuridae), nutcrackers (Nucifraga spp.) and many of insects (e.g., Pyralidae and Tortricidae) exert phenotypic selection on different features of the coniferous cones. In response to such selection, many conifers invest resources in various physical and chemical seed defenses. The study was conducted in mixed mountain forests in West Sudetes (50°55’N 15°46′E), southwest Poland. We selected 10 anvils for great spotted woodpecker and 12 foraging places for the red squirrel. We collected 350 foraged larch cones from each anvil and feeding place for both species and also 50–100 unforaged larch cones within a 15 m radius at each site. In this study, we documented different selection pressures on European larch (Larix decidua) cones exerted by great spotted woodpecker (Dendrocopos major) and red squirrels (Sciurus vulgaris). The great spotted woodpecker preferred foraging on cones with thinner peduncles resulting in selection for thicker peduncles. However, red squirrels preferred feeding on small and medium larch cones, favoring trees producing large cones. Different foraging behaviors influence the form of selection exerted by these two seed-eating species.