Post-fledging recruitment in relation to nestling plasma testosterone and immunocompetence in the spotless starling.

Abstract

1.Social and ecological conditions experienced by individuals during early life can stronglyinfluence their development and survival. Nestlings of many species present important varia-tions in plasma androgens that can be associated with begging and sibling competition andmay translate into fitness effects, since broods with higher testosterone (T) production mayhave better body condition and higher fledging success. However, the positive effects of andro-gens may be counterbalanced by a reduction of immune defences and a greater susceptibilityto diseases.2.In this study we examined the potential relationships between natural variation in plasma T,immunity and post-fledging survival rate in nestlings of the spotless starling (Sturnus unicolor).3.We found that nestlings with higher cellular-mediated immune responses (CMI; measured asa swelling response to phytohaemagglutinin injection) were more likely to be recruited in thepopulation than nestlings with lower CMI responses. Males presented higher CMI response thanfemales, possibly due to differences in competitive advantage over food. We also found thatCMI response was negatively related to T levels, as predicted by the immunocompetence handi-cap hypothesis. However, despite this reduction in CMI response, we failed to find an associa-tion between nestling T levels and survival prospects. Our results add to the evidence of the roleplayed by immune defences in determining survival prospects in natural populations.4.In conclusion, our study reveals that CMI response can be considered as a good predictor ofpost-fledging recruitment. As far as we know, this is the first study attempting to evaluate therelationship between nestling T and post-fledging survival. Our results suggest that the potentialbenefits accrued by high levels of T in sibling competition during the nestling stage do not trans-late into increased survival