When to Return to a Renewing Food Source

Some food sources gradually renew following depletion or depression by a forager. If this renewal process is temporally predictable, then such food sources provide a natural equivalent of a fixed-interval schedule of reinforcement. A forager that returns early to the food source will obtain less food than the maximum possible if it had delayed its return until the source had completely renewed. A forager that returns late to the food source will also obtain less than the maximum rate of food intake possible from that food source because it has waited longer than was necessary to obtain the maximum amount of food, and in situations where there is competition from other foragers, there is the added risk that the food might be lost to a competitor if the forager fails to claim it as soon as it is available. Thus, a forager that can learn the temporal predictability of the food source will be at an advantage over one that cannot, because it can schedule its visits to the source to correspond with the times at which maximum food is available and thus maximize its rate of food intake from the source.

There are several possible natural examples of renewing food sources with predictable temporal properties. For example, in some of the flower species used by nectar-feeding hummingbirds the amount of nectar available in the flower increases predictably and monotonically as a function of the time elapsed since the flower was last visited until the flower has fully refilled. In a field study of long-tailed hermit hummingbirds (Phaethonis superciliosus), Gill (1988) studied the responses of wild hummingbirds to 10- and 15-min FI schedules on an artificial nectar feeder. Rather than filling the feeder gradually, as would occur in a natural flower, he refilled the feeder either 10 or 15 min after the feeder had been emptied by a bird, as in a conventional FI schedule. He showed that under conditions of nearly exclusive use of a feeder by a single individual when the risk of competitive loss was low, return intervals increased to longer than the FI, thereby maximizing the probability of obtaining nectar on a visit.

Another possible example of a renewable food source with temporally predictable properties is provided by the amphipod crustacean Corophium volutator. Corophium species are the major food source of redshanks (Tringa totanus), wading birds that forage on tidal mudflats. The Corophium species live in burrows and retreat down their burrows when a redshank walks over the sediment surface to a depth where they are inaccessible to the redshanks (Goss-Custard, 1970). This is known as prey depression, and measurements of the feeding rates of redshanks have shown that it takes around 10 min for feeding rates to recover following the previous visit by a bird (Yates et al., 2000). This interval is presumed to correspond to the time it takes the Corophium species to resurface from their burrows. There is no information on whether redshanks are able to time this interval, but it would clearly be adaptive for them to do so, because if they return to an area within 10 min of previously feeding there, they will experience a reduced rate of prey capture.

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