Lack of evidence for improved immune response of extra-pair nestlings in collared flycatcher Ficedula albicollis

Extra-pair paternity is common in many socially monogamous bird species. Increasing evidence suggests that extra-pair copulations are female-driven, but benefits for females mating outside social pair-bonds are still poorly understood. The most influential explanation, good genes hypothesis, states that females mated socially with low quality males, engage in extra-pair copulations to obtain genetic benefits for their progeny. According to this model, enhanced performance of extra-pair offspring is expected. Here, based on 4-year study of collared flycatcher Ficedula albicollis, we compared the condition of extra-pair and within-pair young. We found no difference in immune response and body size between maternal half-siblings raised in the same nests. Additionally sex ratio was not biased among extra-pair nestlings, and paternity was not associated with hatching rank. Our results failed to reveal good genes effects in the studied population. These effects might be hard to detect, but other hypotheses should also be studied more thoroughly in the future.