American crows cache less preferred walnuts

Abstract

Animals that can cache food are faced with the options of hiding or eating each food item they encounter. Empirical evidence from a few bird and mammal species suggests that the likelihood of caching may be influenced by a number of factors, including perishability, handling time, food value and the presence of potential cache robbers. American crows,Corvus brachyrhynchos , forage in large flocks for walnuts, and each nut must be either eaten immediately or transported elsewhere for caching. To examine which factors influence caching behaviour in this species, I observed free-living crows foraging for two species of walnuts at naturally occurring and provisioned sites, and documented characteristics of preferred and less preferred walnuts in dichotomous choice tests. The majority of crows flew off with their nuts and cached them in agricultural fields 1–2km away, but others stayed near the walnut trees and cracked their nuts open by dropping them repeatedly on the ground. Nuts that were transported to agricultural fields and cached were significantly smaller than those eaten on site, indicating that crows were being selective about whether to consume or cache nuts. In dichotomous choice tests crows preferred English over black walnuts, which is unsurprising as English walnuts have higher energy value and are easier to crack. When choosing within-nut species, crows preferred cracked over intact and large over small nuts, also consistent with preferences based on minimizing handling costs or maximizing energy content. When provisioned with nuts of different species, sizes and condition (intact or slightly cracked), crows were more likely to cache black and intact walnuts. They were also more likely to cache nuts later in the day. When they did cache the more preferred English walnuts, crows spent more time hiding each nut. These results suggest that crows consumed preferred nuts immediately and cached less desirable nuts.