Stable isotopes predict reproductive performance of European starlings breeding in anthropogenic environments

Understanding reproductive performance in ecologically impoverished vs. more sustained anthropogenic habitats is critical to assess population health status and to develop land use and conservation management strategies. We compared resourcebased maternal effects, reproductive performance, and offspring quality in a model migratory passerine bird, the European starling (Sturnus vulgaris). We assessed female condition, quantity of egg constituents, quality of diet consumed during egg formation, and nestling growth and survival in two habitats: cultivated farmland and meadows. Egg, albumin, and shell mass were greater at the meadow site, while yolk mass did not differ significantly between sites; albumin mass most strongly predicted egg mass. Stable isotope enrichment (δ15N and δ13C) in yolk but not albumin suggests a broader range of diet for yolk formation at the meadow site but could also reflect different hydric conditions between sites. δ13C and δ15N enrichment did not predict yolk, albumin, or egg mass. Concentration of yolk testosterone was higher at the meadow site and correlated with δ13C enrichment in yolk. Nestling survival was higher in the meadow than in the farmland site and corresponded to egg mass and δ13C enrichment in lipidfree yolk. Surviving nestlings were larger in the meadow than in the farmland site. Results indicate that agricultural practice influences reproductive output through resourcebased maternal effects. The analyses of isotopic and biochemical composition of small samples of yolk and albumin may provide a minimally invasive tool to assess individual reproductive performance and predict impacts of habitat quality on population health.