Limited access to food and physiological trade-offs in a long-distance migrant shorebird. II. Constitutive immune function and the acute-phase response.

In response to unbalanced energy budgets, animals must allocate resources among competing physiological systems to maximize fitness. Constraints can be imposed on energy availability or energy expenditure, and adjustments can be made via changes in metabolism or trade-offs with competing demands such as body-mass maintenance and immune function. This study investigates changes in constitutive immune function and the acute-phase response in shorebirds (red knots) faced with limited access time to food. We separated birds into two experimental groups receiving either 6 h or 22 h of food access and measured constitutive immune function. After 3 wk, we induced an acute-phase response, and after 1 wk of recovery, we switched the groups to the opposite food treatment and measured constitutive immune function again. We found little effect of food treatment on constitutive immune function, which suggests that even under resource limitation, a baseline level of immune function is maintained. However, birds enduring limited access to food suppressed aspects of the acute-phase response (decreased feeding and mass loss) to maintain energy intake, and they downregulated thermoregulatory adjustments to food treatment to maintain body temperature during simulated infection. Thus, under resource-limited conditions, birds save energy on the most costly aspects of immune defense.