Because granivorous birds need to exert bite forces to open seeds, evolution resulted in greater bite forces in species feeding on hard seeds. However, beaks also play a role in sound modulation during singing. They are especially important in song fragments with a high trill rate, which require fast open-close movements of the beak. Studies in canaries showed that females prefer songs with high trill rates, implying an important sexual selection pressure on beak speed in this species. In canaries, beak velocities and frequencies are approximately equal during feeding and singing. However, in finches with strong bite forces, like many Darwin’s finches, beak movements during singing are considerably slower than in canaries. Yet, there is no information about the difference in beak velocities and frequencies between feeding and singing in finches with strong beaks. Here we test the hypothesis that, also in species with a strong bite force, song performance is limited by the beak’s movement capacities. This is done using high-speed video recordings of the Java finch (Lonchura oryzivora) during both singing and feeding on seeds. Contrary to our hypothesis, significantly lower beak frequencies occurred during singing compared to during feeding. Although the beak generally rotates at higher amplitudes during singing, our results nevertheless indicate that Java finches are not producing their potential maximum trill rate. Such usage of the beak below its biomechanical potential in terms of speed and frequency during singing suggests different sexual selection pressures compared to species with considerably higher trill rates like canaries.