Abstract
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 abandonment rate (irrespective of clutch size), but mainly in food-poor conditions. This is the first demonstration of parental clutch reduction in anticipation of brood-stage food limitation. Last-egg abandonment functions as an equivalent of abortion, as discarded offspring are excluded from the postnatal selection arena. This strategy might have evolved as ‘best-of-a-bad-job’ to reallocate parental resources when a strong mismatch between clutch size and chick survival probability reduced the hedging value of later-laid eggs.