Abstract Parental care (including postnatal provisioning) is a major component of the offspring’s early-life environment. In avian species, the number of chicks in the nest and subsequent sibling competition for food are known to affect chick’s growth, leading in some cases to long-lasting effects for the offspring. Because of its central role in converting energy, variation in the offspring’s mitochondrial metabolism could be an important pathway underlying variation in growth patterns. Here, we performed a brood size manipulation in great tits ( Parus major ) to unravel its impact on offspring’s mitochondrial metabolism and reactive oxygen species (ROS) production in red blood cells. We investigated the effects of brood size on chicks’ growth and survival, and tested for long-lasting effects on juvenile mitochondrial metabolism and phenotype. As expected, chicks raised in reduced broods had a higher body mass compared to enlarged and control groups. However, mitochondrial metabolism and ROS production were not significantly affected by the treatment either at chick or juvenile stages. Chicks in very small broods were smaller in size and had higher mitochondrial metabolic rates. The nest of rearing has a significant effect on nestling mitochondrial metabolism, yet variation in mitochondrial metabolism at the early-life stages are not associated with survival chances. The contribution of the rearing environment in determining offspring mitochondrial metabolism emphasizes the plasticity of mitochondrial metabolism in changing environments. Further studies would be needed to closely investigate what are the major environmental cues affecting the offspring mitochondrial metabolism during the growth period.

Early-life environmental effects on mitochondrial aerobic metabolism: an experimental brood size manipulation in wild great tits