To test whether alterations in aggressive behavior are responses to short-term resource depression, use of inexpensive and escalated territorial defense by Anna hummingbirds was analyzed as a function of the degree to which food could be depressed by foraging activities of territory owners and intruders. On depressible territories use of gorget display (a low cost defense) decreased and durations of chases increased as time since the previous feeding by owners increased. On non-depressible territories neither variable was significantly correlated with time since feeding. The percentage of intruders chased by owners was positively correlated with time since feeding on both depressible and nondepressible territories, but the magnitude of this trend decreased as food production increased. Departures from the territory tended to occur shortly after feeding on both types of territories, but this tendency was stronger on depressible territories
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