The harsher environment of higher altitudes increases selection for biparental care and increases extrinsic mortality, both of which are expected to reduce the strength of sexual selection. The intensity of sexual selection is often studied using sexual dimorphism as a proxy, especially sexual dichromatism. In birds, multiple studies have shown that sexual dichromatism decreases with increasing altitude. However, those studies have either used small datasets or have used human scoring of sexual dichromatism, potentially missing out on significant levels of cryptic dichromatism. This study includes the measure of subjective sexual dichromatism of the Vorobyev-Osorio colour discrimination model of sexual dichromatism in 758 species of bird with UV-sensitive visual systems and tests whether a relationship exists between altitude and sexual dichromatism. We found a significant positive relation between altitude and sexual dichromatism when accounting for the UV-sensitive vision of Passeriformes, Charadriiformes, Psittaciformes and Struthioniformes, but not when dichromatism is measured using human scoring. This suggests that there might be a greater selection pressure for females to select high-quality males in harsher, higher altitude, environments and that this signalling is primarily done in the ultraviolet range. We suggest that macroecologists should pay closer attention to the receiver psychology of signals even when studying a large number of species and that not doing so could lead to misleading or spurious macroecological and/or macroevolutionary patterns.
