Immune System Responses to Ectoparasite Infections in Nestling Barn Swallows Hirundo rustica: an Experimental Approach

In altricial [helpless] birds, ectoparasite [external parasite] exposure during development may elicit a costly immune response requiring nestlings to redistribute energy away from growth and development and towards parasite defense. That altricial nestlings are born with an immature immune system may have implications for the mechanisms they use for immune defense. I studied how nestlings defend themselves against ectoparasites as well as how nestling immune defense interacts with nestling mass. I specifically examined how innate versus acquired immune responses react to ectoparasite exposure by assessing leukocyte [white blood cell] proportions as well as how the different branches of the immune system interacted with nestling mass. Nestlings experimentally exposed to parasites during development tended to have higher proportions of heterophils [innate leukocyte] and lower proportions of lymphocytes [acquired leukocyte] compared to nestlings not exposed to ectoparasites. When exposed to ectoparasites, the acquired immune system response required a greater body mass than the innate immune system response; nestlings with higher body mass exhibited a higher proportion of lymphocytes and had a lower proportion of heterophils. My findings aid in the understanding of how altricial nestlings mount an immune response to ectoparasites as well as lay a foundation for future studies exploring how the costs of defense against ectoparasite may interact with somatic [the body's] growth and immune system development.