Data from: 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
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.