Parental and genetic factors rather than nesting forest patch size affect physiological traits of offspring in an area‐sensitive passerine: an experimental study

Our understanding of the effects of habitat loss on individual performance is limited due to a lack of experimental studies that take the potential genetic and parental effects producing phenotypic variation into consideration. To assess the relative role of habitat loss on offspring phenotype while controlling for the confounding effects of genetic and parental variation we performed a partial cross‐fostering experiment using the Eurasian Treecreeper Certhia familiaris. The experiment was carried out by swapping half the nestlings in a brood between small and large nesting forest patches to determine the effect of nesting forest patch size on five nestling traits reflecting morphological size, body condition, physiological stress and inflammation status. There was no effect of nesting forest patch size on the offspring traits examined. Instead, we found evidence of genetic and early parental effects on all traits except inflammation status, as well as parental effects after cross‐fostering for all of the measured offspring traits. Our results suggest that genetic and parental effects should be taken into account when making inferences about species’ responses to habitat loss.