The Cost of Phenotypic Mismatch in Avian Energy Intake: Limited Spare Capacity in Extreme Phenotypes Is Compensated by Energy-Saving Mechanisms.

AbstractDaily energy intake in animals is flexible but takes days to adjust when needs surpass capacity. To buffer mismatches between intake and demand, individuals maintain immediate spare capacity for food consumption. The declining spare capacity model posits that this capacity declines as traits approach maximal values. In contrast, the high midrange capacity model suggests that animals may maintain higher spare capacity at intermediate levels between baseline and maximal capacity when facing challenging environments. We tested these models by examining how captive pine siskins (Spinus pinus) acclimated to -5°C, 5°C, and 20°C responded to a 10°C temperature drop. Within 72 h, food intake increased by 22% in birds acclimated to 20°C, 14% in birds from 5°C, and 6% in birds from -5°C, with a treatment effect persisting for 37 d. Despite their highest spare capacity, 20°C-acclimated birds lost nearly three times more fat than those acclimated to -5°C. However, -5°C birds compensated by increasing lean mass and recovered 53% of their initial fat loss. These results support the reduced spare capacity model but also show that although cold-acclimated birds have limited immediate capacity to increase intake, they can still defend and restore energy reserves during mismatches between intake and demand, likely through physiological trade-offs.