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Structural colors result from the physical interaction of light with organic materials of differing refractive indexes organized at nanoscale dimensions to produce significant interference effects. Because color properties emerge from these finely organized nanostructures, the production of structural coloration could respond to environmental factors and be developmentally more plastic than expected, functioning as an indicator of individual quality. However, there are many unknown factors concerning the function and mechanisms regulating structural coloration, especially relative to social environment. We hypothesized that social environment, in the form of competitive settings, can influence the developmental pathways involving production of feather structural coloration. We experimentally assessed the impact of social environment upon body condition, molt and spectral properties of two types of structural color that compose the nuptial plumage in blue-black grassquits: black iridescent plumage and white underwing patches. We manipulated male social environment during nine months by keeping individuals in three treatments: (1) pairs; (2) all-male groups; and (3) male-female mixed groups. All morphological characters and spectral plumage measures varied significantly through time, but only acquisition of nuptial plumage coverage and nuptial plumage color were influenced by social environment. Compared with males in the paired treatment, those in treatments with multiple males molted into nuptial plumage faster and earlier, and their plumage was more UV-purple-shifted. Our results provide experimental evidence that social context strongly influences development and expression of structural plumage. These results emphasize the importance of longterm experimental studies to identify the phenotypic consequences of social dynamics relative to ornament expression. [ABSTRACT FROM AUTHOR]