Hormonal response of nestlings to predator calls

Chicks of altricial birds may perceive predation risk by acoustic cues produced by predators. This capacity involves changes to a less conspicuous behavior to avoid being detected and predated. However, little is known about the physiological mechanisms underlying this capacity. Potential mechanisms may involve corticosterone and testosterone, two hormones related to begging, an acoustic signal, which could be used by predators to locate and predate the nest. However, given the species-specificity of this relationship, it is difficult to make particular predictions. We manipulated perceived risk of nest predation in the common blackbird (Turdus merula) and analyzed nestlings' plasma levels of these hormones to look for nest predation risk effects. Our experimental manipulation showed hormonal changes in response to nest predation risk. Chicks under a high risk of nest predation reduced their corticosterone plasma levels but increased their testosterone levels in comparison with nestlings exposed to a low nest predation risk. We explain our results as mechanisms to reduce begging activity and discuss them within the framework of hormonal modifications in developing animals. These findings highlight the importance of studying nest predation from the unusually considered chick perspective and underlined the benefits from including physiological variables in the study of predator-prey interactions.