Captive and free-living red knots Calidris canutus exhibit differences in non-induced immunity that suggest different immune strategies in different environments

Experiments on captive animals, in which conditions can be controlled, are useful for examining complex biological phenomena such as immune function. Such experiments have increased our understanding of immune responses in the context of trade-offs and pathogen pressure. However, few studies have examined how captivity itself affects immune function. We used microbial killing, leukocyte concentrations and complement-natural antibody assays to examine non-induced (constitutive) immunity in captive and free-living red knots Calidris canutus. Univariate and multivariate analyses indicated that captive and free-living birds differed in their immune strategies. Captive birds showed reduced S. aureus killing, C. albicans killing, heterophils, and eosinophils. In a principal component analysis, the affected variables fell onto a single axis, that reflected phagocytosis and inflammation based immunity. We discuss possible reasons for this result in an immune cost and protective benefit framework.