Oscine song diversified into a great variety of phonology and syntax across species. This diversity is bounded by a combination of physiological constraints to song production. The range of sounds that each species is able to produce can be thought of as a multidimensional space, and within that space, closer to its limits, it may be physiologically demanding to sing. Some studies compared sound amplitude among syllables to infer which traits may be more demanding to sing loudly. We measured sound amplitude of syllables within songs of 24 species of Serinus, and found that in 19 species some traits were consistently sung less loudly. This suggests that it is not uncommon that some syllable traits are more demanding than others to sing loudly. Across species, the more elements composite syllables had, the stronger its negative effect on loudness, suggesting repeated evolution of a costly trait. These results suggest that the evolution of song phonology and syntax is influenced by how loudly different species need to sing.