Assessing the role of reproduction and stress in the spring emergence of haematozoan parasites in birds.

A spring emergence of avian haemosporidian infections is nearly universal among temperate zone birds and is often described as a cost of reproductive effort. We take advantage of the opportunistic (i.e. aseasonal) breeding schedule of the red crossbill (Loxia curvirostra) to determine the relative contributions of season versus host physiology to the timing and intensity of Haemoproteus infections in the temperate zone. Despite breeding activity in both the winter and summer, Haemoproteus infections were highly seasonal--occurring largely from May through September--and measures of host physiology (i.e. reproductive condition and stress parameters) did not explain parasite prevalence. However, within the spring-summer peak, infection intensity (i.e. parasite density) was positively correlated with plasma levels of testosterone and free corticosterone and negatively correlated with corticosterone binding globulin capacity. These data are discussed in terms of the behavioral ecology of host and vector, and suggest that both seasonal increases in vector activity and relapse of latent (i.e. dormant) infections contribute to the spring emergence in birds. Relapse of latent infections does not appear to be induced by reproductive activity or increased allostatic (i.e. energy) load, but rather by a season-specific change in host or parasite physiology (e.g. melatonin or endogenous rhythms).