Are brood sex ratios adaptive? – The effect of experimentally altered brood sex ratios on parental feeding behaviour

Abstract

Correlations between brood sex ratios (BSRs) and parental or environmental quality have been found in many species. This phenomenon is called sex ratio adjustment, and is expected to evolve if certain factors affect the fitness return from the offspring in a sex-dependent way. However, it is seldom studied whether biased sex ratios are indeed adaptive. We manipulated BSRs in a cross-fostering experiment, and investigated parental costs in terms of feeding rate and survival in the collared flycatcher (Ficedula albicollis). In our population, male nestlings can grow faster under good conditions, but are more sensitive to adverse conditions. Assuming that the sensitivity of the males results from their larger energy requirement, we predicted increased costs in broods with male-biased experimental BSR. Assuming that BSR adjustment is adaptive and related to parental care giving capacity, we expected higher feeding and survival rate by parents that originally had more sons, and predicted that low quality parents are less able to adjust their feeding rates to the needs of their foster broods or pay higher survival cost. However, we found that the manipulated BSR and its interaction with original BSR affected neither the feeding rate nor the survival of the parents. Only male feeding rate was correlated with original BSR, however, contrary to our prediction: males with female-biased original BSR fed their foster chicks more frequently. Our results, with those of a previous report about the effects of the experiment on nestlings, do not support that the observed BSRs are adaptive in our population.

Significance statement

Many hypotheses propose that higher vertebrates adaptively adjust the primary sex ratio of their offspring to individual or environmental quality. While the potential adaptive value of the observed patterns is regularly discussed, studies that specifically test the adaptivity of sex ratio adjustment are very scarce and correlative. Using a special cross-fostering experiment, we investigated whether original brood sex ratios are related to the rearing capacity of the parents, and experimental sex ratios are related to the rearing costs in terms of feeding effort or survival. We found no effect of experimentally altered brood sex ratios on either parental feeding effort or survival. Furthermore, contrary to the adaptive scenario, males that had female-biased broods originally had higher feeding rates. So far, we have found no evidence that the sex ratio adjustment is adaptive in the collared flycatcher.