The role of olfaction in birds is poorly understood, in part because of our limited knowledge of signal-transmission mechanisms. Here we evaluate the hypothesis that the secretions of the uropygial gland serve as olfactory signals in birds by testing the prediction that size of the olfactory bulb, a proxy for olfactory ability, covaries positively with the size of the uropygial gland's tuft, a circlet of feathers whose size varies extraordinarily from species to species. The function of this tuft has remained a mystery, but mechanical or protective roles are unlikely on the basis that these feathers are downy and always saturated with gland secretion. These observations instead suggest that the tuft may be involved in trapping the compounds produced by the gland's secretions to facilitate conspecifics' perception of odor. We therefore predicted that the uropygial gland's tuft should be more developed in birds with better capacity to smell. Using a dataset of 29 species of birds of 20 families of nonpasserines, we show that the sizes of the tuft (relative to the uropygial gland) and olfactory bulb (relative to the cerebral hemisphere and body mass) are positively correlated after the confounding effects of colonial breeding and phylogeny are controlled for. This suggests that the tuft may have evolved because of the adaptive benefits of enhancing the transmission of body odors. Additionally, colonial species have tufts larger than those of solitary species, as expected because sociality increases encounter rates and the prevalence of odor-producing bacteria.