The evolution of honest animal communication is mostly understood through the handicap principle, which is intrinsically dependent on the concept of individual quality: low-quality individuals are prevented from producing high-quality signals because if they did so, they would pay greater production costs than high-quality individuals. This study tested an alternative explanation for the black bib size of male house sparrows Passer domesticus, an honest signal of quality whose expression is negatively related to levels of the pigment pheomelanin in its contituent feathers. Authors previously showed that experimental depletions of cysteine, which participates in pheomelanogenesis, improve the phenotype (bibs larger than controls) of high-quality males (birds with largest bibs initially) only. Here an experiment was conducted under opposite conditions, increasing the availability of dietary cysteine, and obtained opposite results: deteriorated phenotypes (bibs smaller than controls) were only expressed by high-quality birds. Some birds were also treated with the pro-oxidant diquat dibromide, and it was found that the cellular resistance to free radicals of high-quality birds benefited more from the antioxidant activity of cysteine against diquat than that of low-quality birds. These findings support the existence of a mechanism uncoupling cysteine and pheomelanin in low-quality birds that confers them a low sensitivity to variations in cysteine availability. This constitutes an explanation for the evolution of signal honesty that overcomes the limitations of the handicap principle, because it provides a specific definition of individual quality and because costs are no longer required to prevent low-quality individuals from producing large signals. informacion[at]ebd.csic.es: Galván & Alonso-Alvarez Individual quality as sensitivity to cysteine availability in a melanin-based honest signalling system. J Exp Biol doi: 10.1242/jeb.160333