Self-sustained vibrations within the syrinx of birds are simulated using symmetric two-mass models. The first system that will be presented here is a rescalcd version of the well-known model of human vocal fold vibrations. Moreover, a novel model of the syrinx of a ring dove is introduced. We show that the intensity of harmonics depends strongly on the geometry of the vibrating tissue and the driving pressure. This leads to a discussion how birds can control the strength of overtones.