Separating the genetic and environmental drivers of body temperature during the development of endothermy in an altricial bird

When altricial birds hatch, they are unable to regulate their own temperature, but by the time they fledge they are thermally independent. Early-life conditions have been shown to be an important factor contributing to fitness. However, it is currently unknown to what extent body temperature during endothermy development is driven by genetic variation or by the early environment. We use thermal images of cross-fostered house sparrows (Passer domesticus) throughout the nestling period to separate genetic and environmental drivers of body temperature. We estimated negligible heritability of body temperature at all ages. We further found that there are effects from the natal environment that carry over into the late nestling stage. A correlation between the early- and mid-nestling periods was explained by the natal environment, and during this period body temperature and growth followed independent developmental trajectories. Furthermore, higher body temperature was under viability selection, independent of body mass. We, therefore, demonstrate that the natal environment influences future offspring phenotype via a novel measure; body temperature. Our study provides a novel investigation into the environmental and genetic drivers of body temperature variation in a wild bird, furthering our understanding of how traits evolve.