Birds' choices concerning nest attendance are influenced by state-dependent and environmental variables including food availability and predation pressure. Parents of altricial young with biparental care face a dilemma: they can choose to guard their nestlings against predation with their physical presence at the nest and ensure the fledging of at least one chick, or they can choose to leave the young unguarded so they can forage for more food, increasing provisioning rates and the probability of fledging more young. In group-nesting birds, parents may be able to dilute the risk of nest predation while leaving the nest to forage if they leave the nest unguarded when there are many other unguarded nests in the colony. The complexity of the parental decision to leave a nest unguarded is highlighted by variation in the chicks' age, within a nest and within the colony, and by differences in effects on nest success and nest fecundity. Our results suggest that the incremental fitness advantage of reducing predation risk by continuing to guard the nest is eventually outweighed by opportunities to reduce nestlings' risk of starvation. The effect is likely to reduce fledgling production within a colony when less food is available, which reinforces the importance of healthy and productive wetland ecosystems in maintaining viable populations of the Great Egret.