Adaptive management of body mass by Siberian jays

► Body mass was measured repeatedly without capture in wild Siberian jays. ► Daily increases in mass during autumn were linear irrespective of social class. ► As predicted, mass increased as conditions became colder and more variable. ► Mass was adjusted to wind chill 24 h earlier to match current conditions. ► Mass of dominants was less dependent on body size than mass of subordinates.