Social environment affects testosterone level in captive male blue-black grassquits

►We tested how social setting changes testosterone level and aggression in blue-black grassquits. ► Males were submitted to three social conditions during 1 year: all males, mixed, and paired. ► We quantified aggressiveness among males and T plasma for individuals in each treatment monthly. ► There was more aggressiveness in the all-males treatment and T patterns varied across treatments. ► Male perception of social environment ultimately mediates hormonal production.