RESUMEN
Although random meiosis should prevent the facultative adjustment of offspring sex ratio, theory predicts that females should produce more of the sex with the higher reproductive value. We reported a case of offspring sex ratio manipulation in grass wrens Cistothorus platensis. Males in better body condition would have higher reproductive value than females due to the potential for social polygyny and extra-pair fertilizations. On the other hand, local demography influences reproductive strategies in grass wrens as male abundance affects both social polygyny and extra-pair paternity frequencies. We evaluated whether females bias their brood sex ratio in response to adult sex ratio and nestling body condition (a proxy for female's prospects of producing high-quality males). Females raised more male offspring when males were less abundant in the population (female-biased adult sex ratio). However, we found no relationship between nestling body condition and brood sex ratio, suggesting that females did not bias the brood sex ratio towards males when able to raise nestlings in better body condition. Taken together, our results provide the first suggestive evidence that female birds can manipulate their offspring sex ratio in response to the adult sex ratio.
