The ‘small bird in winter’ paradigm states that body mass is a balance between the conflicting demands of carrying enough energy to survive nightly fasts while minimizing the risk of predation associated with carrying additional fat reserves. We conducted a short-term food-supplementation experiment during which New Zealand robins (Petroica australis) were provided with food on the second day of a 3-d trial. This allowed us to test two predictions from models of strategic mass regulation in small birds: (1) individual birds reach the same end-of-day mass despite differences in their initial morning mass while, (2) using surplus energy for increased singing. As expected, robins gained mass at a higher rate early in the morning on the fed day than they did on either of the two control days, but there was no significant difference in their evening masses across the 3 d of the experiment despite birds on day 3 starting at higher initial masses than birds on day 1. Robins displayed a significantly higher rate of singing when receiving food supplements on day 2, supporting a link between energetic reserves and behavior. Our results suggest that potentially energetically costly behaviors, such as song production, are sensitive to short-term changes in energy reserves, and that both state and behavioral predictions can be successfully integrated to provide tests of state-based models of behavior.