Trade-off between immune response and body mass in wintering house sparrows (Passer domesticus)

Immunity is necessary in order to fight parasites and pathogens, but immune protection also incurs a cost for the hosts. Therefore, immunity should be traded against other fitness-related traits. Body mass, as a function of body reserves, is important in birds of temperate zones during winter. Sedentary temperate birds usually increase body mass just before winter to survive a period with low food availability and thermoregulatory requirements. Given that immunity is costly and affects body mass, a trade-off is predicted between body mass and immunity in wintering birds. Such a trade-off was tested experimentally by stimulating the immune system of a group of wintering house sparrows (Passer domesticus) from a Spanish population in an outdoor aviary with a lipopolysaccharide while another group served as control. The activation of the immune system negatively affected body mass, despite the relatively mild temperatures of the study zone. Such a trade-off between body mass and immunity was found in other similar studies during breeding, but not during moult, suggesting that the trade-off between immunity and body mass may vary seasonally.