Field studies have shown that animals often abandon territorial defense when food is abundant, but the causes of this behavior are controversial. Sometimes the cessation of defense is attributed to the food supply being so abundant that monopolization of the resource gains the defender nothing, even though defense would be energetically feasible. Other studies show that when the food supply is rich, such large numbers of competitors are attracted that defense is overwhelmed. Knowledge of the amount of food available relative to the numbers of consumers in the region can resolve the controversy. When food is limited throughout a region, a localized rich foraging area will attract competitors.Defense of such an area therefore may yield a large increase in net benefit to the defender, depending on whether or not so many competitors recruit that defense is too costly. However, when a localized foraging area is rich but food is also superabundant throughout the region, intruders may not recruit even if the area is undefended. Therefore the benefits of defense, if it were to occur, would be low. Field studies of nectar-feeding birds show that defense continued on rich foraging areaswhen floral nectar was limited to the population regionally, and resulted in enhanced food availability relative to that in undefended areas. However, under conditions of artificially rich food supplies (sugar-water feeders) and extremely limited food regionally, defense was sometimes overwhelmed by competitors. Cessation of defense on locally rich foraging areas also occurred when nectar was regionally superabundant. Therefore, both proposed causes of cessation of defense occurred in these systems and were easily distinguished by examining the degree to which food was limited in the surrounding region relative to localized foraging areas. I show that animals potentially can assess whether or not food supply is limited in the surrounding region by sampling and comparing standing crops in defended and undefended situations.

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