Brood sex ratios (BSRs) have often been found to be non-random in respect of parental and environmental quality, and many hypotheses suggest that non-random sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross-fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated...