Associations between immune parameters, parasitism, and stress in breeding pled flycatcher (Ficedula hypoleuca) females

There are two major interpretations of serum IgY concentration in wild birds. On the one hand, it has been considered an indication of susceptibility to stress and parasite infection. Therefore, immunoglobulin concentration is expected to change in response to variation in these factors owing to reproductive activities. On the other hand, it has been considered a measure of immune capacity. We measured the IgY level and the lymphocyte proportion at the beginning of incubation and at the end of the nestling period in female pied flycatchers, Ficedula hypoleuca (Pallas, 1764). We assessed the immune response to phytohaemagglutinin (PHA) at the latter stage. We found that the total IgY level remained constant throughout the season. Initially, it was positively associated with the PHA response, lymphocyte proportion, intensity of infection by Haemoproteus spp., and concentration of stress protein HSP70 in peripheral blood. These variables explained nearly 80% of the variation in IgY concentration. In the final phase, only the PHA response was correlated with the IgY level. We discuss the hypothetical mechanisms underlying these associations and the need to control for parasite infection and physiological stress in ecological studies including measurements of immunoglobulin concentration.