Effects of mesotocin manipulation on the behavior of male and female common waxbills

•We manipulated the avian oxytocin homolog, mesotocin, in a highly social bird.•Birds treated with mesotocin shoswed a sedative state.•Birds treated with an oxytocin antagonist showed a decrease only in social behaviors.•Sex-specific effect: mesotocin reduced allopreening more in males than females.•Sex-specific effect: oxytocin antagonist reduced aggressiveness only in females. The oxytocin family of neuropeptides is implicated in the regulation of sociality across vertebrates. Non-mammalian homologs of oxytocin, such as isotocin in fish and mesotocin in amphibians, reptiles and birds, all play crucial roles modulating social and reproductive behavior. In this study, we exogenously manipulated the mesotocinergic system in a highly social bird, the common waxbill Estrild astrild, and tested the effects on affiliative and aggressive behavior by performing tests of competition over food. Birds treated with mesotocin decreased almost all the behaviors we studied (movement, feeding, allopreening), while birds treated with an oxytocin antagonist showed a reduction only in social behaviors (aggressions and allopreening). We also found two sex-specific effects: mesotocin reduced allopreening more in males than females, and the oxytocin antagonist reduced aggressiveness only in females. Our results suggest sex-specific effects in the modulation of affiliative and aggressive behaviors via mesotocinergic pathways.