Avian prey-dropping behavior. II. American crows and walnuts

Complex and energetically expensive foraging tasks should be shaped by natu ral selection to be efficient. Many species of birds open hard-shelled prey by dropping the prey repeatedly onto the ground from considerable heights. Urban-dwelling American crows (Corvus brachyrhynchos) forage in this way o n two species of walnuts in central California, USA. As predicted from a th eoretical model, crows dropped nuts with harder shells from greater heights and dropped them from greater heights when over softer substrates. The hei ght selected for dropping nuts decreased in the presence of numerous nearby conspecifics, indicating that crows were sensitive to the risk of kleptopa rasitism when selecting drop heights. Drop height decreased with repeated d rops of the same walnut, suggesting that crows adjusted for the increasing likelihood that a repeatedly-dropped nut would break on subsequent drops. C rows did not alter height of drop in accordance with differences in the mas s of the prey. When faced with multiple prey types and dropping substrates, and high rates of attempted kleptoparasitism, crows adjusted the height fr om which they dropped nuts in ways that decreased the likelihood of kleptop arasitism and increased the energy obtained from each nut.