Secondary sexual characters have been hypothesized to reveal the ability of males to resist debilitating parasites. Although such reliable signaling of parasite resistance may be maintained by parasite--host coevolution, maternal effects potentially provide a previously neglected factor that could affect the level of genetic variation in resistance to parasites. That could be the case because maternal effects have an entirely environmental basis, or because they can maintain considerable amounts of genetic variation through epistatic effects, even in the presence of strong directional selection. Maternal effects have been shown to occur as maternal allocation of immune factors to offspring, and such allocation may depend on the mating prospects of sons, causing mothers to differentially allocate maternal effects to eggs in species subject to intense sexual selection. Here we show that a maternal effect through innate antibacterial immune defense, lysozyme, which is transferred from the mother to the egg in birds, is positively associated with the evolution of secondary sexual characters. Previous studies have shown that females differentially allocate lysozyme to their eggs when mated to attractive males, and elevated levels of lysozyme are associated with reduced hatching failure and superior health among neonates and adults. In this study, comparative analyses of lysozyme from eggs of 85 species of birds showed a strong positive relationship between brightness of male plumage and egg lysozyme, even when controlling for potentially confounding variables. These findings suggest that maternal immune factors may play a role in the evolution of secondary sexual characters. Copyright 2007, Oxford University Press.