Fighting Behavior in Bald Eagles: A Test of Game Theory

Seven predictions of evolutionary game theory were examined in field studies of foraging behavior of Bald Eagles (Haliaeetus leucocephalus) wintering in the Chilkat Valley, Alaska. A cost/ benefit analysis revealed that the frequencies of two foraging strategies (hunting and stealing from conspecifics) were balanced such that the payoffs of the two were nearly equal. Asymmetries in probable correlates of fighting ability (size and, possibly, spatial position [being in the air vs. on the ground], but not age) and expected gain in victory (hunger level) influenced the outcome of contests over food. Individuals used conditional strategies: small or young birds appeared to hunt (rather than steal) relatively more often than others. Pirating eagles often assessed the size and hunger level of food defenders and attacked those most likely to retreat. Contrary to prediction, ritualized displays served to advertise expected gain in victory and were good indicators of subsequent behavior. The level of escalated fighting was inversely related to resource availability. Finally, a graphical model shows that pirating frequency may or may not be influenced by changes in food abundance. The results generally support the predictions of game theory and explain several aspects of Bald Eagle foraging behavior.