Paradoxical calls: the opposite signaling role of sound frequency across bird species

The behavioral literature contains inconsistent results on the function of sound frequency (pitch) across species, offering an unexplored opportunity to investigate evolutionary diversification of communication systems. I review those results for birds, where about half the studied species use lower than average frequency (LAF) as a relevant sexual signal, and the remaining species use higher than average frequency (HAF) for the same functions. This variation appears nonrandom with respect to putative causal factors, suggesting that advertising body size determines which species use LAF as a sexual signal. I evaluate different hypotheses to explain why the remaining species use HAF instead. Integrating tests of alternative hypotheses on focal species will be required to demonstrate the causes for this divergence in communication systems.