Disentangling the mechanisms of signal evolution in Tyrannidae flycatchers, part I: song is constrained by morphology and covaries with ecological factors

. Acoustic signals mediate key animal interactions and can evolve through a variety of factors. Signal divergence can reinforce pre-zygotic barriers and minimize costly hybridizations among closely related species or partition acoustic space to avoid signal interference. To unravel the drivers of song evolution, it is critical to simultaneously test multiple evolutionary axes leading to heterospecific song variation (e