An avian equivalent of selective abortion: post-laying clutch reduction under resource limitation

Selective elimination of excess offspring with poor fitness prospects may occur prenatally (selective abortion) or postnatally (brood reduction). Postnatal reduction is the dominant strategy, presumably because surplus progeny serves as a hedge against environmental and developmental uncertainty. In birds, its main proximate mechanism is asynchronous hatching, generating within-brood competitive asymmetry. Here, clutch-size reduction via last-egg abandonment was investigated in the asynchronously hatching red-necked grebe in a study area comprising two human-managed poorly predictable habitats with distinctly different food supplies. Last-egg abandonment, virtually absent in favorable food conditions, occurred regularly in larger clutches in conditions of brood-stage food scarcity. In the food-poor habitat, the production and body condition of fledglings did not differ between last-egg abandoning and caring pairs. The experimentally prolonged hatching interval increased the egg abandonm...