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

Very few studies address the effect of hardness on seed selection in graniv orous birds. As a defense against predators, plant species may produce seed s of various hardnesses, some of which are too hard for a bird to crack. Un successful 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 forc e of the bird, it is difficult to distinguish between seed selection as a r esult of mechanical constraints and selection as a result of preference. Ou r experiments aimed to discriminate between these two effects. During two s eries of experiments, Java Sparrows (Padda oryzivora) were offered safflowe r seeds. Size characters and hardness of the seeds that remained after feed ing 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 forc e were eaten, with the rest being rejected. After some experience, birds st arted to actively select for seed size (e.g., depth) and preferred to eat t he smallest seeds. Although the correlation between size and hardness is lo w, the birds successfully used size characteristics as a predictor for hard ness.