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Behavioral Ecology Vol. 10 No. 3: 220-226
© 1999 International Society for Behavioral Ecology

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

Daniel A. Cristola and Paul V. Switzerb

a Department of Biology, The College of William and Mary, Williamsburg, VA 23187-8795, USA, b Department of Zoology, Eastern Illinois University, Charleston, IL 61920, USA

Address correspondence to D.A. Cristol. E-mail: dacris{at}facstaff.wm.edu

Complex and energetically expensive foraging tasks should be shaped by natural 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 on two species of walnuts in central California, USA. As predicted from a theoretical model, crows dropped nuts with harder shells from greater heights and dropped them from greater heights when over softer substrates. The height selected for dropping nuts decreased in the presence of numerous nearby conspecifics, indicating that crows were sensitive to the risk of kleptoparasitism when selecting drop heights. Drop height decreased with repeated drops of the same walnut, suggesting that crows adjusted for the increasing likelihood that a repeatedly-dropped nut would break on subsequent drops. Crows did not alter height of drop in accordance with differences in the mass of the prey. When faced with multiple prey types and dropping substrates, and high rates of attempted kleptoparasitism, crows adjusted the height from which they dropped nuts in ways that decreased the likelihood of kleptoparasitism and increased the energy obtained from each nut.

Key words: American crow, Corvus brachyrhynchos, foraging behavior, Juglans spp., kleptoparasitism, prey dropping, walnuts.


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