Winder, L. A.; Hogger Gadsby, J.; Wellman, E.; Pick, J. L.; Simons, M. J. P.; Burke, T.

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 an individual\'s performance in adult life. 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. Our results show small heritability of body temperature. We further found that there are effects from the natal environment which carry over into the late nestling stage. A correlation between early to the mid-nestling period was explained by the natal environment and during this period body temperature and growth follow independent developmental trajectories. Furthermore, higher body temperature is under viability selection independent of body mass. We, therefore, demonstrate the natal environment influences future offspring phenotype in a novel measure; body temperature. Our study provides the first study into the environmental and genetic drivers of body temperature variation in a wild bird, furthering our understanding of how an individual\'s traits evolve.