Components of phenotypic variation in avian ornamental and non-ornamental feathers

Phenotypic variation, measured as the coefficient of variation (CV), is usu ally larger in secondary sexual characters than in ordinary morphological t raits. We tested if intraspecific differences in the CV between ornamental and non-ornamental feather traits in 67 evolutionary events of feather orna mentation in birds were due to differences in (1) the allometric pattern (s lope of the regression line when regressing trait size on an indicator of b ody size), or (2) the dispersion of observations around the regression line . We found that only dispersion of observations around the regression line contributed significantly to total variation. A large dispersion of observa tions around the regression line for ornamental feathers is consistent with these characters showing condition-dependence, supporting indicator models of sexual selection more strongly than a pure Fisher process. Ornamental f eathers in males demonstrated negative allometry when regressed on tarsus l ength, which is a measure of skeletal body size. This finding is consistent with ornamental feather traits being subject to directional selection due to female mate preferences, where large body size is less important than in male-male competition. This pattern of phenotypic variation for avian seco ndary sexual characters contrasts with patterns of variation for insect gen italia, supposedly subject to sexual selection, since the latter traits onl y differ from ordinary morphology traits in allometry coefficient. The prev ailing regime of selection (directional or stabilizing) and the effects of environmental factors are proposed to account for these differences among t raits.