Activation of an immune response (IR) upon exposure to pathogens is crucial to ensure adequate organismal performance and is directly linked to survival. Fitness benefits of the response may be associated with costs in terms of increased energy expenditure and may compete for resources and compromise such fitness benefits. Trade‐offs between immune function and other traits relevant for fitness are well documented, however, it remains unknown if such trade‐offs are energetically mediated. We manipulated the flight activity of 70 zebra finches Taeniopygia guttata to investigate the energy reallocation to the immune system in rested and exercised birds. Four experimental groups exhibiting different flight intensity were used: trained, untrained and birds that either stopped or started flight training after the immune challenge. If costs associated with the IR and flight activity compete for energy, we predicted the extent of inhibition of IR would be dependent on energy allocated to physical activity. Daytime resting metabolic rate was measured before and after the immune challenge, induced using sheep red blood cells (SRBC). Strength of the response was measured as the concentration of anti‐SRBC antibodies six days post‐challenge. We found no evidence for the predicted inhibition of the immune function between trained and untrained birds, as there was no difference in resting metabolic rate between experimental groups. However, resting metabolic rate following the challenge was negatively correlated with the IR. Surprisingly, individuals with relatively low resting metabolic rates following immune challenge were able to up‐regulate their IR, indicating a trade‐off in the use of the energy resource independent of flight activity levels. Our results suggest that energy allocation to mount the IR may represent a constraint that is possibly linked to the circadian pattern of the energy budget but appears to be independent of energetic challenges brought on by different levels of flight activity.
