The term 'marginal value' in foraging refers to:
The marginal value theorem evaluates the rate at which a forager gains energy while exploiting a depleting patch. Early in a visit, abundant accessible resources produce rapid gains; as the patch is depleted, the instantaneous gain rate falls. Departure is favored when that marginal rate declines to the average rate obtainable across the environment after accounting for travel time between patches. Thus the relevant quantity is energy gained per unit time, not energy per bite or the identity of a least-preferred prey. The theory predicts longer residence in richer patches and when travel is co
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 10