Experimental warming of embryos accelerates metabolism of postnatal songbirds, influences growth, but has no effects on parental feeding and brooding rates

Here we investigate possible carryover effects of experimentally increased incubation temperatures for postnatal growth, metabolism, and parental care within and among 6 north temperate and one tropical songbird species. Increased temperatures during embryonic development consistently caused higher postnatal resting metabolism compared to control nests, among but not within the 7 species studied. The effects of the experiment on growth were species specific and depended on the morphometric considered. Size before the fledge date was positively correlated with feeding rate, and metabolism was lower in larger broods. Our experiment did not elicit changes in parental food delivery rates or brooding effort during the postnatal stage, and higher brooding effort was associated with nestlings of smaller mass and faster metabolism independently from treatment. Consequently, parental care seemed unlikely to be the cause of the differences in growth rates between treatments. Instead, physiological mechanisms triggered by our heating treatment appear to be responsible for the observed variation in growth. These intrinsic changes unmatched by adjustments in parental effort may contribute to longer-term consequences for individual quality and survival that deserve further attention. LAY SUMMARY Temperatures experienced during incubation can have later influences on the growth and metabolism of chicks. Temperatures additionally impact the energy balance of parents and therefore their ability to allocate resources for feeding themselves and their brood after hatching. This evidence suggests that early temperature conditions during the egg stage can have downstream effects on nestling growth that can be fully understood only by integrating physiological and parental contributions. After experimentally heating nests of 7 different species of songbirds, we measured nestling metabolism and growth, together with parental rates of feeding and brooding. Our results show that nestlings from heated eggs had increased metabolism and some species changed their growth trajectories, but this was not associated with different rates of parental brooding and feeding. These results are important because higher ambient temperatures are forecasted all over the world and may influence thermal conditions during incubation. However, growth sustained by higher food availability is considered beneficial, though when associated with accelerated metabolism is considered detrimental for individual