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 phyto haemagglutinin 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 o f sampling. We assessed this relationship experimentally by measuring the c hanges in body mass and CMI in individual captive yellow-legged gulls (Laru s cachinnans) maintained with food provided ad libitum (control group), wit h no food (fasting group), or with one-third of their daily food requiremen ts (subfeeding group). We identified a direct, nonlinear relationship betwe en food intake, body mass, and CMI. Before the experiment started, body mas s of birds (corrected for size) fed ad libitum did not correlate with their CMI, while a positive correlation was found after food restriction. This s uggests that birds may reach a threshold above which increases in food inta ke and body mass do not enhance CMI. Thereafter, food restriction caused de creases in CMI that were significantly correlated with the percentage of bo dy mass lost by each bird. However, for birds that lost similar. proportion s of body mass, changes in CMI varied according to food-restriction treatme nt, the subfeeding group exhibiting a stronger CMI than the fasting group.