Seed selection in the Java Sparrow (Padda oryzivora): preference and mechanical constraint

Abstract

Very few studies address the effect of hardness on seed selection in granivorous birds. As a defense against predators, plant species may produce seeds of various hardnesses, some of which are too hard for a bird to crack. Unsuccessful cracking attempts lead to loss of time and, thus, lowers energy-intake rate. Birds may prefer seeds with a short handling time and a large chance of being cracked. However, without knowing the maximal cracking force of the bird, it is difficult to distinguish between seed selection as a result of mechanical constraints and selection as a result of preference. Our experiments aimed to discriminate between these two effects. During two series of experiments, Java Sparrows (Padda oryzivora) were offered safflower seeds. Size characters and hardness of the seeds that remained after feeding were compared with a control group. Without prior experience, the birds showed selection as a result of mechanical constraints. Seeds were chosen randomly and only seeds with a hardness less than the maximal crushing force were eaten, with the rest being rejected. After some experience, birds started to actively select for seed size (e.g., depth) and preferred to eat the smallest seeds. Although the correlation between size and hardness is low, the birds successfully used size characteristics as a predictor for hardness.