Preference and Profitability: Theory and Experiment

Foraging theory (Krebs et al. 1983; Stephens and Krebs 1986) assumes a simple hierarchy of feeding decisions. It supposes that foragers first choose among habitats, then choose among patches within habitats, and finally choose among prey items within patches. These three decisions have usually been studied independently. In this paper we begin to integrate these problems by considering diet choice within patches both experimentally and theoretically. Our theoretical arguments follow from combining well-known ideas about diet choice (Pulliam 1974; Charnov 1976a) and patch use (Charnov 1976b; Orians and Pearson 1979). We consider patches that are nothing more than clumps of discrete prey, and we ask which prey should be attacked and in what order? Despite the simplicity of this approach, there is some confusion in the literature about how foragers should choose prey within patches. Waddington and Holden (1979; see also Waddington 1982) pointed out that conventional diet theory (Pulliam 1974; Charnov 1976a) assumes that the forager encounters prey items one after the other (sequential encounter). Waddington and Holden argued that many foragers experience simultaneous prey encounters (e.g., a visual hunter may see many items at once) and that this fact will change the diet theory's predictions. Waddington and Holden presented a rule for choosing among simultaneously encountered items. Their rule characterizes a prey item by the quotient of its net energy value (e) divided by the amount of time (11) required to pursue and handle it. We use' 'pursuit" to mean the same thing as Waddington and Holden's term "travel"; like Waddington and Holden, we mean the time from encounter until the start of actual handling. This quotient (e/h) is the familiar "profitability of optimal-diet theory (Pulliam 1974; Charnov 1976a). Throughout this paper we use the term "profitability" only in this restricted sense. Waddington and Holden argued that the "best" a rate-maximizing forager can do when simultaneously encountering items is to attack the item with the highest profitability (see fig. lA). We call this the take-the-most-profitable rule. The take-themost-profitable rule claims that the relative profitabilities of the items encountered