Nonlinear Biomechanical Resonances in Birdsong

Evolution has shaped animal bodies, yet to what extent biomechanical systems impose constraints and provide opportunities across different behaviors remains unclear. In birds, quiet breathing operates at a resonance of the respiratory biomechanics, but song, a behavior thought to be shaped by strong sexual selection, requires much higher breathing rates. Combining physiological recordings with a nonlinear biomechanical model, we show in canaries (Serinus canaria) that song production drives the system into a nonlinear regime that broadens the frequency range of amplified responses. This enhancement encompasses the full range of syllabic rates, with an average magnification of ~94% of the theoretical maximum. Thus, birds sing at a resonance, indicating that rapid song rhythms evolved to operate under shifting natural frequencies of the respiratory biomechanics. Our results illustrate a shared optimization strategy across behavioral states, reveal a deep connection between neural and biomechanical dynamical parameters and show that sexually selected displays may still rely on optimization strategies.