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

Complex and energetically expensive foraging tasks should beshaped bynatural selection to be efficient. Many species ofbirds open hard-shelledprey by dropping the prey repeatedlyonto the ground from considerableheights. Urban-dwelling Americancrows (Corvus brachyrhynchos) foragein this way on two speciesof walnuts in central California, USA. As predictedfrom a theoreticalmodel, crows dropped nuts with harder shells from greaterheightsand dropped them from greater heights when over softer substrates.Theheight selected for dropping nuts decreased in the presenceof numerous nearbyconspecifics, indicating that crows weresensitive to the risk ofkleptoparasitism when selecting dropheights. Drop height decreased withrepeated drops of the samewalnut, suggesting that crows adjusted for theincreasing likelihoodthat a repeatedly-dropped nut would break on subsequentdrops.Crows did not alter height of drop in accordance with differencesinthe mass of the prey. When faced with multiple prey typesand droppingsubstrates, and high rates of attempted kleptoparasitism,crows adjusted theheight from which they dropped nuts in waysthat decreased the likelihood ofkleptoparasitism and increasedthe energy obtained from each nut.