Variation within populations is a pre-requisite for the action of selection on morphological traits. Darwin assumed that there was much greater variation in sexual ornament size than in body size, but this may not be generally true of natural populations. I analyse field data on variation in body size and the length, area and mass of tail ornaments in paradise (Vidua paradisaea) and shaft-tailed whydahs (V. regia). Whydahs are promiscuous, brood parasitic African finches with elaborate tail ornaments in breeding males. The short, unadorned tails of male shaft-tailed whydahs, which carry a wire-like tail ornament, are non-significantly (1%) longer than female tails, but male paradise whydahs, which carry a large, broad ornament, have unadorned tails 10% longer than those of conspecific females. Fully grown ornament length, mass and area vary little more (CVs = 1.8−6.4%) than male or female body size traits (CVs = 1.7−6.1%). Instead, there is high variation in the timing of ornament development during prenuptial moult (CVs = 30.8−39.5% for paradise whydahs and 12.6−23.8% for shaft-tailed whydahs when corrected to a standard date). This temporal variation in development probably has greater significance for sexual selection in whydahs than maximum ornament size. Keywords: Ornament size, body size, sexual selection, sexual dimorphism, age dimorphism, morphological variation, aerodynamics, Vidua paradisaea, Vidua regia.