Effects of experimental food restriction and body-mass changes on the avian T-cell-mediated immune response

The T-cell-mediated immune response (CMI) of birds, measured with the phytohaemagglutinin skin test, is in most cases positively correlated with their body mass. This correlation, however, does not imply causality, since high-quality birds may be more immunocompetent as well as heavier at the time of sampling. We assessed this relationship experimentally by measuring the changes in body mass and CMI in individual captive yellow-legged gulls (Larus cachinnans) maintained with food provided ad libitum (control group), with no food (fasting group), or with one-third of their daily food requirements (subfeeding group). We identified a direct, nonlinear relationship between food intake, body mass, and CMI. Before the experiment started, body mass of birds (corrected for size) fed ad libitum did not correlate with their CMI, while a positive correlation was found after food restriction. This suggests that birds may reach a threshold above which increases in food intake and body mass do not enhance CMI. Thereafter, food restriction caused decreases in CMI that were significantly correlated with the percentage of body mass lost by each bird. However, for birds that lost similar. proportions of body mass, changes in CMI varied according to food-restriction treatment, the subfeeding group exhibiting a stronger CMI than the fasting group.