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 optimization hypothesis (IOH), Yellow-headed Blackbird (Xanthocephalus xanthocephalus) clutches of three and four eggs were experimentally enlarged or reduced by one egg, and broods were monitored until hedging. Predictions that control clutches would a) have greater hedging success than experimental clutches, and b) produce heavier or bigger offspring than enlarged broods were generally not supported. Within control clutches, the number of nestlings that fledged did not increase with clutch size. In fact, three-egg clutches were more productive than the most common clutch size of four eggs. The IOH appears to be unsupported for this population, at least up until the time of fledging. We suggest a potential insurance value exists for the fourth-laid egg, thus explaining the maintenance of a modal clutch size of four eggs in this species.