Experimental manipulation of maternal effort produces differential effects in sons and daughters: implications for adaptive sex ratios in the blue-footed booby

Sex allocation theory predicts that mothers in good condition should bias their brood sex ratio in response to the differential benefits obtained from increased maternal expenditure in sons and daughters. Although there is well-documented variation of offspring sex ratios in several bird species according to maternal condition, the assumption that maternal condition has different fitness consequences for male and for female offspring remains unclear. The blue-footed booby (Sula nebouxii) is a sexually size-dimorphic seabird, with females approximately 31% heavier than males. It has been reported that the sex ratio is male biased in years with poor feeding conditions, which suggests that either females adjust their sex ratio in accordance with their condition or that they suffer differential brood mortality before their sex can be determined. In this study I tested whether the condition of mothers affected their daughters' fitness more than their sons' fitness. I manipulated maternal investment by trimming the flight feathers and thereby handicapping females during the chick-rearing period. Adult females in the handicapped group had a poorer physical condition at end of chick growth, as measured by mass and by the residuals of mass on wing length compared to control birds. Female chicks were affected by the handicapping experiment, showing a lower mass and shorter wing length (reduced approximately 8% in both measures) than controls. However, this effect was not found in male chicks. Hatching sex ratios were also related to female body condition at hatching. The brood sex ratio of females in poor condition was male biased but was female biased for females in good condition. Overall, these results suggest that the variation in the sex ratio in blue-footed boobies is an adaptive response to the disadvantage daughters face from being reared under poor conditions.