Seasonal sex ratio trend in the European kestrel: An evolutionarily stablestrategy analysis

We present an evolutionarily stable strategy (ESS) model to analyze selecti on on seasonal variation in the brood sex ratio, as observed in several spe cies of raptorial birds. The model is specifically tailored to the life his tory of the European kestrel, and it reflects the maturation time hypothesi s, the idea that a seasonal sex ratio trend has evolved because of sex diff erences in the dependence of age of first breeding on date of birth. First we show how to derive a fitness function in the context of a seasonal envir onment. Model parameters are estimated from field data in order to derive q uantitative predictions. Since little is known about constraints on sex rat io control in birds, we analyze three scenarios, each corresponding to a di fferent strategy set. We consider a model without constraints on sex ratio control, a model where the sex ratio trend is constrained to be linear, and a mechanistic model incorporating a plausible mechanism of sex ratio contr ol in birds. One of the models yields an ESS sex ratio trend that closely r esembles the trend observed in the field. However, the predictions are very sensitive to the choice of strategy set. Moreover, the selective forces ge nerated by sex differences in maturation are rather weak. In fact, the mech anistic model shows that seemingly negligible costs of sex ratio control ma y be sufficient to overcome the adaptive value of adjusting the sex ratio.