Maternal immunoglobulin concentration in Collared Flycatcher (Ficedula albicollis) eggs in relation to parental quality and laying order

1. The immune system of newly hatched birds is relatively immature; therefore pathogens can be particularly virulent. Females transfer passive immunity in terms of immunoglobulins to the eggs to protect their young against infections in the crucial early life stages. 2. As transmission of antibodies is likely to be nutritionally costly, mothers are expected to allocate these components differentially to eggs according to their own condition, the quality of their mate or the laying order of eggs. 3. We found that in Collared Flycatchers (Ficedula albicollis), yolk antibody levels positively correlated with female body condition, but showed no relationship with maternal age or body size. Furthermore, females with higher plasma heterophil : lymphocyte ratio and heterophil count, which may indicate higher level of stress, deposited lower amount of immunoglobulins to their eggs. These results suggest that females of better physiological condition were able to invest more immunoglobulins to their eggs. 4. Neither the plumage characteristics, nor the age of the male parent was related to yolk antibody concentration, and thus no evidence for differential allocation of antibodies in relation to male quality or attractiveness could be detected. 5. Last-laid eggs contained higher yolk immunoglobulin concentration than earlier-laid eggs within a clutch. This pattern could be interpreted as a way to improve the survival probability of the disadvantaged last-hatching nestling.