I hypothesize that predation on newborn young represents a seasonal analog to time-lagged predator–prey cycles and can cause directional selection against late birthing. Newborn offspring represent an irruption of vulnerable prey to which predators can respond by adapting their search efforts to find and capture these prey. Those born early would be the 1st to achieve the size or mobility necessary to escape predators. Simple models of predation on moose calves were used to demonstrate how changes in predator efficiency as newborns appear, combined with a short period of offspring vulnerability, would produce selection against late birthing. Of the 9 published studies of mammals, 6 showed evidence of selection against late-born young that could be driven by predation on neonates. If true, this hypothesis has consequences for the way we interpret evidence for “predator swamping” and optimal birthing periods in mammals and other taxa with synchronous reproduction.