CLUTCH-SIZE MANIPULATIONS IN THE YELLOW-HEADED BLACKBIRD - A TEST OF THE INDIVIDUAL OPTIMIZATION HYPOTHESIS

To test the critical short-term predictions of the individual optimiza tion hypothesis (IOH), Yellow-headed Blackbird (Xanthocephalus xanthoc ephalus) clutches of three and four eggs were experimentally enlarged or reduced by one egg, and broods were monitored until hedging. Predic tions that control clutches would a) have greater hedging success than experimental clutches, and b) produce heavier or bigger offspring tha n enlarged broods were generally not supported. Within control clutche s, the number of nestlings that fledged did not increase with clutch s ize. In fact, three-egg clutches were more productive than the most co mmon clutch size of four eggs. The IOH appears to be unsupported for t his population, at least up until the time of fledging. We suggest a p otential insurance value exists for the fourth-laid egg, thus explaini ng the maintenance of a modal clutch size of four eggs in this species .