The effects of the minimum threshold condition for breeding on offspring sex-ratio adjustment in the lesser kestrel

We proposer a model for sex-ratio adjustment complementary to that of Trive rs and Willard. In addition to the three basic assumptions of the Trivers-W illard model, our model assumes that the sex with more variable reproductiv e success (normally male) is also the sex less constrained for reproduction . This assumption seems realistic, because several studies have demonstrate d that poor-condition males may adopt alternative mating strategies and sir e some offspring, whereas females have physiological constraints for gestat ion or egg production that cannot be avoided. Thus, under these circumstanc es, sons of both poor and good condition would be more valuable for parents than daughters, whereas daughters would be relatively more valuable than s ons at intermediate condition. This model predicts, therefore, a U-shaped r elationship between parental condition and offspring sex ratio. We present a case study for the monogamous lesser kestrel (Falco naumanni) that fulfil ls the assumptions and predictions of the model. The minimum body condition for breeding, measured as pectoral thickness, was lower for sons than for daughters. Below this minimum, males had a higher chance of breeding than f emales. Above this minimum, however, the lifetime reproductive success was condition dependent in males but not in females. Thus, males in better body condition attain, on average, higher reproductive success than females. Of fspring sex ratio varied with the size of the father's ornaments and mother condition according to the U-shaped pattern predicted by the model.