Nest morphology correlates with female ornamentation and depredation risk in Diamond Firetails (Stagonopleura guttata)

Female ornamentation as a signal of mate quality has been historically understudied. In bird species where females build the nest, nest morphology – a repeatable and heritable trait in some taxa – may function as an extended phenotypic signal subject to natural or sexual selection. In Diamond Firetails (Stagonopleura guttata), females differ in their ornamentation, measured as the number of white flank spots (‘spottiness’), a trait linked to dominance, reproductive investment, and immune strength. Female Diamond Firetails are the sole builders of the nest, and nests often differ in their morphology, notably in the length of the entrance tunnel. We aimed to investigate the relationship between the phenotype of the nest builder, clutch size and nest architecture, and how nest architecture affects depredation risk. We analysed female morphology (mass, tarsus length, spottiness), nest morphology (nest height, nest length, entrance tunnel length), and clutch size in captive and wild Diamond Firetails. We found that spot number was significantly repeatable in captive birds. Larger females and spottier females in both the wild and in captivity laid larger clutches, and spottier captive birds built nests with longer tunnels. Using clay eggs in artificial nests with manipulated tunnel lengths in the wild, we found that nests with longer tunnels were less likely to be depredated, particularly by birds. These results suggest that nest morphology may function as an extended phenotype related to female ornamentation, both of which can signal female quality and/or investment, and may be selected for through both sexual and natural selection.