The expression of elaborated displays provides reliable information to conspecifics about the quality of the signaler. Competition for breeding resources or mates is predicted to affect the expression of signals in both males and females; however, the literature has been typically focused on male behaviors. The spotless starling is an interesting example where both sexes decorate the nest to signal their condition and social status: males add green plants at the beginning of the breeding period, and females place foreign feathers during the incubation period. In this study, we investigate nest decoration by males and females in relation to the intensity of conspecific competition during the breeding period. We distributed nest boxes at either a high (HD) or a low density (LD) and recorded the amount of green plant material added before laying, the number of feathers at hatching, and the breeding productivity under these different density treatments. The amount of green plant material and the number of feathers at hatching were higher in HD than in LD nests. Furthermore, nest boxes at higher density had lower breeding productivity. Our results suggest that manipulating the density of nest boxes increased competition for breeding resources. The aggregation of males competing for territories and mates may stimulate them to add greater amounts of plants, and competition over those males or other resources for reproduction may induce females to add feathers. This study shows that social environment has the potential to influence the expression of signaling behaviors in both males and females.