The effect of seed hardness on husking time in finches

Small granivorous birds crack and remove the seed coat before they swallow the kernel. It is generally assumed that husking time is related to seed hardness and bite force, although direct experimental evidence is scarce. In this study we decreased experimentally the hardness of a single seed species, so that all the seed characters remained the same with the exception of (average) hardness. We determined the husking time for experimental and control seeds in a number of granivorous passerines. Our data show that husking time is directly related to seed hardness: husking time increases with seed hardness. A video-analysis of the seed-cracking process shows that species also apply different numbers of mandibulations to crack the two seed types. The number of seed positioning movements before cracking slightly increases with the size of the seed relative to body size. The largest contribution to differences in husking time among different sized species or between seeds of the same size but of different hardness, however, comes from the number of cracking attempts. It is hypothesised that seeds are squeezed from between the mandibles more easily when relatively large bite force is applied, leading to an increase in failed cracking attempts.