Birds can adaptively regulate body mass in response to the cost-benefit trade-off between risks of starvation and predation. Despite this, little is known about how birds actually monitor, detect, and interpret their body mass and the mechanisms employed to regulate or adjust body mass to match their current ecological conditions. Using zebra finches, we experimentally increased perceived mass via attachment of weighted backpacks and provided the birds with either ad libitum standard mixed-seed diet or supplementary high-fat to investigate (a) how birds assess their own body mass and (b) the mechanisms birds employ to adjust mass. In both experiments, independent of diet treatment, birds with backpacks rapidly lost mass and reduced overall activity, time spent in active behaviors and food intake. We found no evidence that mass loss was due to stress associated with backpack attachment - there was no difference in plasma corticosterone concentrations directly after rapid mass loss. We found evidence that birds likely interpret body mass via a physical mechanosensory pathway rather than a physiological pathway - rapid loss of mass was not linked to changes in plasma metabolites (glycerol or triglyceride concentrations). Our results suggest that the process of energy balance and mass regulation can involve a greater array of mechanisms than simply matching energy in (food consumed) to energy out. Zebra finches are able to force a negative energy balance through other, unidentified, mechanisms, even while maintaining high-fat dietary intake and reducing overall activity.