To study natural selection, the Grants have observed various traits in finches over the course of many years. One example is beak size. The medium ground finche has a relatively small crushing beak, allowing it to more easily feed on small tender seeds. The Grants quantified beak size among the medium ground finches on Daphne Major by carefully measuring beak depth (a measurement of the beak from top to bottom, at its base) on individual bird (Figure 3). The small size of the island made it possible for them to measure a large percentage of birds and their offsprings. During the course of their study, they compared the beak size of parents and offspring by examining many broods over several years. They found that the size of the beak was transmitted from parents to offspring, regardless of environmental conditions, indicating that differences in beak sizes are due to genetic differences in the population. In other words, they found that beak size was a heritable trait.