Tracing site‐specific isotopic signatures along a Blue Tit Cyanistes caeruleus food chain

Food chains culminating with temperate insectivorous passerines are well described, yet whether trophic webs can be site-specific remains a largely unexplored question. In the case of site- or habitat-specificity of food webs, stable isotope signatures of bird feathers may enable assignment of unmarked individuals to a site or a habitat of origin. We address this question in landscapes that include contrasting forest habitat patches with either deciduous Downy Oak Quercus humilis or evergreen Holm Oak Quercus ilex as dominant tree species. First, we examine the spatial variation across habitats and sites in the stable isotope ratios of carbon (d13C) and nitrogen (d15N) along the oak leaf-Tortrix moth Tortrix viridana caterpillar-Blue Tit Cyanistes caeruleus food chain. Secondly, we assess whether the isotopic signatures allow for correct assignment of individual birds to their site or habitat of origin. At the scale of the landscape, stable isotope values enabled identification of the different components of the Blue Tit food chain: from oak leaves to Blue Tit nestlings and yearling birds. However, isotopic signatures were site-specific (i.e. geographical) more than habitat-specific (i.e. deciduous vs. evergreen oaks). Discriminant analyses correctly assigned 85% of nestlings and 83% of resident yearling birds, indicating a pronounced effect of site on Blue Tit feather isotopic signatures. We thereby demonstrate that isotopes reflect a stronger association of locally born birds to the local features of their habitat than that of un-ringed yearling birds, whose plumage may have grown while in a wider geographical area. This study provides evidence of site-specific isotopic signatures from oak leaves to Blue Tit feathers at a fine spatial scale.