Nest decoration as a social and sexual signal in a brood-deserting songbird

Sexual selection frequently acts on multiple signals, including both individual ornaments and extended phenotypes. Previous studies in birds indicate that nest decorations can function as extended phenotype signals to either conspecifics or sexual partners. However, it is still unclear how extended phenotypes interact with body ornaments and simultaneously mediate social and intrapair interactions. In this study, we investigated the dual social and intrapair roles of nest decorations and how they combine with body ornaments to shape parental investment and reproductive success in a wild population of rock sparrows, Petronia petronia . Female rock sparrows decorate the nest and both sexes are ornamented with a conspicuous yellow breast patch. We found that highly decorated nests were not randomly distributed in space but tended to occur in more densely clustered breeding areas, suggesting that nest decorations may be influenced by the local social environment. Moreover, nest decoration and male breast patch size were positively associated with male, but not female, parental investment. Path analyses further revealed that the effect of nest decoration on nestlings' fledging success was entirely indirect and mediated through male parental care, suggesting that nest decorations function as an intrapair signal linked to paternal investment. Our results suggest that nest decorations may serve a dual function in both social (internest) and intrapair contexts and highlight the importance of multiple ornaments as a mechanism shaping parental investment strategies.