An immune challenge affects growth dynamics, oxidative stress and survival in wild spotless starling nestlings.

The activation of the immune system in response to a pathogen infection can impose significant costs on growth and oxidative stress. Developing individuals are particularly vulnerable to this challenge, as their physiological systems are still maturing and their rapid growth to achieve an optimal size is resource demanding. Here, we investigated the costs in terms of growth and oxidative stress of an experimentally induced immune challenge in wild spotless starling (Sturnus unicolor) nestlings. To this aim, individuals were injected with a lipopolysaccharide (LPS), an antigen that triggers an immune response, or a control substance (PBS) in a within-brood design. Although the immune activation impaired the rate of body mass gain within the first 6h after the challenge, nestlings subsequently showed an accelerated increase in mass within the following 18h, reaching a similar body mass to the Controls by the next day. This represents a compensation ('catch-up growth') occurring within as little as 6h to 24h after the challenge. Despite this compensation, initially smaller challenged nestlings showed reduced body mass and survival rates by 8 days after treatment compared to smaller Control nestlings. The induced immune challenge also led to increased plasma levels of oxidative damage (reactive oxygen metabolites, ROMs) and antioxidant capacity (OXY), supporting the oxidative cost of immune system activation. These findings highlight the costs of mounting an immune response during early life, characterized by an altered growth dynamic and increased oxidative damage, and the condition-dependence of these effects.