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Avian Prey-Dropping Behavior. II. American Crows and Walnuts

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Avian Prey-Dropping Behavior. II. American Crows and Walnuts Behavioral Ecology Vol. 10 No. 3: 220±226 Avian prey-dropping behavior. II. American crows and walnuts Daniel A. Cristola and Paul V. Switzerb aDepartment of Biology, The College of William and Mary, Williamsburg, VA 23187-8795, USA, and bDepartment of Zoology, Eastern Illinois University, Charleston, IL 61920, USA Complex and energetically expensive foraging tasks should be shaped by natural selection to be ef®cient. 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 conspeci®cs, 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. [Behav Ecol 10:220±226 (1999)] irds that break open hard-shelled prey items by dropping dictions of our model on a previously undescribed avian prey- B them repeatedly onto the ground have attracted scien- dropping system. ti®c attention for at least a century (e.g., Saunders, 1891). American crows, Corvus brachyrhynchos, which drop two spe- This foraging behavior is more complex than most because it cies of walnuts (English, Juglans regia, and northern California requires repeated ¯ights with a prey item, choice of drop sites black, J. hindsii), presented an ideal opportunity to test our and drop heights, and retrieval of prey. Such a demanding model. We ®rst gathered background data on the heights from task is ideal for studying how closely an animal's behaviors are which we had to drop each type of walnut until it was cracked adapted to the rigors of its environment. Because avian prey- suf®ciently to be consumed by a crow and data on the effects dropping occurs in at least three distantly-related orders of of different substrates, nut mass, and repeated impacts on the birds (Table 1), this behavior has presumably evolved a num- breakability of the walnuts we dropped. We also determined, ber of times. The occurrence of avian prey-dropping on nearly by observing foraging crows, whether the probability of a crow every continent, in a wide-range of habitats, and with many losing its walnut was dependent on the height from which the different types of prey suggests the potential for interesting nut was dropped. These data allowed us to choose a speci®c adaptive modi®cations (Table 1). Finally, the occurrence of set of predictions from our general theoretical model that prey dropping among both highly gregarious and essentially could be applied to the crow±walnut system (Switzer and Cris- solitary species raises the question of whether, when, and how tol, 1999). To test these predictions we gathered data on the this complex behavior is learned. heights from which crows dropped walnuts in situations where Much of the early interest in avian prey-dropping behavior important factors could be varied. For example, we observed arose from debate over whether some birds (crows) selected crows feeding simultaneously on two walnut species that dif- ef®cient hard surfaces for dropping, whereas others (gulls) fered markedly in the ease with which they could be broken, were less ``intelligent'' because they reportedly failed to dis- thereby allowing us to test predictions about the effects of prey criminate between hard and soft substrates (Oldham, 1930; hardness on the height selected for dropping. Tinbergen, 1960). More recently authors have attempted to A central prediction of our model was that the presence of understand avian prey-dropping in the context of optimal for- prey loss should have dramatic consequences on dropping be- aging (e.g., Barash et al., 1975; Beck, 1980; Maron, 1982; Sieg- havior (Switzer and Cristol, 1999). Although prey loss and theft fried, 1977; Whiteley et al., 1990; Zach, 1978, 1979). In our are mentioned in most previous studies of prey dropping (e.g., companion paper (Switzer and Cristol, 1999) we presented a Barash et al., 1975; Siegfried, 1977; Whiteley et al., 1990; Zach, general theoretical model that identi®es important variables 1979) few quantitative data are available. In the present study, that should shape all avian prey-dropping systems. The model crows frequently stole walnuts dropped by other crows, and suggests that the height from which hard-shelled prey will be dropping occurred under varying degrees of kleptoparasitism dropped is in¯uenced by the hardness of the prey item, the intensity. Thus, we were able to examine, for the ®rst time, the hardness of the substrate onto which it is dropped, the mass effects of prey loss on this complex foraging behavior. of the prey item, the potential weakening of the shell during repeated drops, and the likelihood of loss after dropping. The objective of the present study was to test the qualitative pre- CHARACTERIZING WALNUTS Methods Address correspondence to D.A. Cristol. E-mail: dacris@facstaff. Before making speci®c predictions using the model presented wm.edu. in our companion paper, it was necessary to understand the Received 30 September 1997; accepted 12 September 1998. effects of height, prey type, substrate hardness, repeated q 1999 International Society for Behavioral Ecology drops, and prey mass on the probability of a prey item break- Cristol and Switzer x Crows and walnuts 221 Table 1 Bird species and the prey they drop Prey types (no. of Bird species reported prey species) Reference Corvus brachyrhynchos (American crow) Nuts (3) Switzer and Cristol, 1999; Grobecker and Pietsch, 1978; Maple, 1974 C. caurinus (northwestern crow) mollusks (3) James and Verbeek, 1984; Richardson and Verbeek, 1986, 1987; Zach, 1978 C. corone cornix (hooded crow) Mollusks (4), nuts Berrow et al., 1992; FoÈrstel, 1993; Lorek and Oleksik, 1992; Mienis, 1993 (3), crustaceans C. c. corone (carrion crow) Mollusks (2), nuts (2) Nihei, 1995; Oldham, 1930; Saunders, 1891; Tinbergen, 1960; Whiteley et al., 1990 C. frugilegus (rook) Mollusks Priestley, 1947 C. monedula (jackdaw) Nuts Gibson, 1992 C. moneduloides (New Caledonian crow) Nuts Hunt, 1996 C. corax (common raven) Mollusks, bones Heinrich, 1989; van Lawick-Goodall, 1970 C. albicollis (white-necked raven) Turtles Uys, 1966, cited in Goodwin, 1986 Gypaetus barbatus (lammergeier) Bones, turtles Hartley, 1967; Huxley and Nicholson, 1963 Neophron percnopterus (Egyptian vulture) Turtles Lesham, 1979, cited in Boswall, 1983 Pandion haliaetus (osprey) Mollusks Lesham, 1979, cited in Boswall, 1983 Haliaetus leucocephalus (bald eagle) Turtles Bindner, 1968 Aquila chrysaetos (golden eagle) Turtles citations in Boswall, 1983 Catharcta skua (South polar skua) Eggs Sladen, 1958, cited in Boswall, 1977 Larus argentatus (herring gull) Mollusks (8), ®sh, Beck, 1980; CadeÂe, 1989; Cavanagh, 1992; Connor, 1993; Ingolfsson and crustaceans (4), Estrella, 1978; Kent, 1981; Oldham, 1930; Tinbergen, 1960 echinoderms (2) L. delawarensis (ring-billed gull) Mollusks (3) Drennen, 1995 L. canus (mew gull) Mollusks Oldham, 1930 L. dominicanus (kelp gull) Mollusks (.2) Siegfried, 1977; Ward, 1991 L. glaucescens (glaucous-winged gull) Mollusks (2) Barash et al., 1975 L. marinus (great black-backed gull) Mammals, eggs Harber and Johns, 1947; Tinbergen, 1960 L. melanocephalus (Mediterranean gull) Mollusks (2) van Impe, 1978 L. occidentalis (western gull) Mollusks (5) Maron, 1982 L. paci®cus (Paci®c gull) Mollusks (2), Teichert and Serventy, 1947; Wheeler, 1946 echinoderms ing open on a given drop (Switzer and Cristol, 1999). We (1979), we used linear regression to determine whether either determined these characteristics for walnuts by dropping relationship was positive. A positive slope would indicate weak- them from an exterior stairway at various heights. ening with successive drops. This was repeated for 100 black To determine the effect of height on the probability of a walnuts of similar weights dropped from 10.85 m, which is a dropped walnut breaking, we dropped 20 English walnuts comparable height in terms of the number of drops required onto asphalt at each of the following heights: 1.55, 3.1, 6.2, to crack these harder nuts. Black walnuts exhibited a similar and 9.3 m. Each nut was dropped repeatedly from the same pattern to English walnuts (data not shown), but because we height until its shell had cracked open suf®ciently to be con- have insuf®cient data to examine the behavior of crows suc- sumed by a crow (throughout the paper ``cracked'' refers to cessively dropping black walnuts (see below), there was no a hole of at least 1 cm2). The same procedure was repeated reason to include black walnuts in this analysis. The same with black walnuts from 3.1, 6.2, and 9.3 m. A two-factor (spe- black and English walnut data were used to determine the cies, height) ANOVA was used to compare the number of relationship between number of experimental drops required drops required to break English and black walnuts dropped to break a walnut and its mass. from these three heights. To determine the effect of substrate hardness on the prob- Results ability of a dropped walnut breaking, English walnuts were dropped from 6.2 m and 9.3 m into a container of hard- By dropping nuts experimentally, we found that the number packed soil from an agricultural ®eld used by crows for nut of drops required to crack a walnut decreased with height of dropping.
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