Corticosterone effects on aggression in a passerine species, the common waxbill Estrilda astrild

Glucocorticoids mediate behavioral and physiological responses to environmental stressors, supporting individuals' survival and fitness. Corticosterone, as the primary avian glucocorticoid, modulates these physiological responses. We focus on the common waxbill Estrilda astrild, a highly social and gregarious passerine that feeds and flocks together year-round, providing an excellent model to explore the impact of glucocorticoids on social behavior, especially in aggression. To investigate this, we conducted a correlational study in captive birds, examining baseline corticosterone plasma levels and three behavioral assays: 1) the tonic immobility test, a standard indicator of fear; 2) the mirror test, an assay to assess the reactive-proactive personality axis; and 3) food competition tests, which evaluate social behavior and aggression. Subsequently, we experimentally manipulated the common waxbill corticosterone levels by using exogenous high and low dosages of corticosterone to induce a short-term glucocorticoid challenge and evaluate its effects on behavior. Our results revealed that birds with higher baseline corticosterone plasma levels were less aggressive, but an acute increase in corticosterone levels increased aggression. Furthermore, corticosterone receptor antagonist reduced activity but did not affect feeding. These results suggest that corticosterone can influence aggression and activity in common waxbills, leading to a role in regulating social behavior in both sexes of this gregarious passerine species.