What size and weight of Manchurian Walnuts Juglans mandshurica should crows consume?

Abstract

Carrion Crows Corvus corone drop Manchurian Walnuts Juglans mandshurica onto the ground in order to be able to break them open and eat their contents. Previous studies of avian prey-dropping behavior allow us to predict that the crows make a range of decisions when foraging in this way. They must first choose their walnuts then choose the drop site and the drop height, in order to raise the efficiency of their foraging behavior. To understand this decision making process, we must first understand the characteristics of walnuts. In this investigation we had four objectives: 1) to establish from what height walnuts must be dropped in order to break them. 2) to determine whether the weight of walnuts affects their likelihood of breaking when dropped from a certain height. 3) to understand the relationship between walnut size and weight and the weight of the edible contents. 4) To confirm whether crows really choose walnuts. Our results were as follows: 1) Nine out of thirty (30.0%) walnuts broke when dropped 10 times from a height of 12.7 m, whereas three (10.0%) broke when dropped from 8.8 m, and only one (3.3%) broke when dropped from 4.8 m. 2) The weight of the walnuts was independent of their breakability. 3) There were positive correlations between the above two sets of variables. 4) Crows preferentially chose heavier walnuts.