Immunoglobulin plasma concentration in relation to egg laying and mate ornamentation of female barn swallows (Hirundo rustica)

In vertebrates, offspring have a relatively inefficient immune system soon after birth. Female birds transmit immunoglobulins to the egg, which can co nfer protection against parasites to their offspring after hatching, but al location of immune factors can depend on the reproductive value of the offs pring as affected, for example, by the quality of their father. We analyse the variation in immunoglobulin levels of female barn swallows (Hirundo rus tica) during the breeding cycle in relation to the expression of a secondar y sexual character of their mates. Circulating immunoglobulins peaked on ap proximately the day before that of laying of their first egg, but postlayin g concentration was similar to the concentration well before laying. Immuno globulin levels per unit volume of plasma were lower for females breeding l ate compared with those breeding early. Haematocrit of females reached an a bsolute minimum on the day of laying of the third egg. In males, concentrat ion of immunoglobulins relative to other plasma proteins did not change in relation to the breeding stage. Smaller relative concentrations of immunogl obulins and haematocrit were observed in males breeding late in the season. Immunoglobulin concentration of females was positively correlated with the level of ornamentation of their mates. These results suggest that females alter their immune profile to transmit humoral factors providing immune def ence against pathogens to their offspring after hatching. This enhancement of immune protection is larger when the offsprings are of relatively large reproductive value as when sired by high quality males. Alternatively, fema les with relatively large immunoglobulin concentration preferentially mate with the most ornamented males.