The nesting biology of Australia’s endemic Diamond Firetail (Stagonopleura guttata) is poorly known. However, Diamond Firetails are known to weave flowers into the entrance of their nests sometimes, although the function of this behaviour is not known. This study used field and aviary data to address the following questions: (1) What proportion of nests in the wild have flowers woven into the nest-entrance? (2) Are nests with flowers more successful than nests without flowers? (3) Does the vegetation of the nest-site predict nesting outcome? (4) How often do wild birds reuse nests across years for breeding? (5) Do birds have a preference for stem-length or presence of flowers when selecting nesting material in aviaries? The field data showed that 70% of nests had flowers woven into the entrances, but with significant annual variation in the number of flowers at nest-entrances, with > 30 flowers per nest in 2004 and < 10 flowers per nest in 2005. Most (96%) natural nests were depredated (100% in 2004, and 91% in 2005), and we could not examine the role of vegetation of the nest-site in determining predation outcome. Using artificial nests with and without flowers we found no effect of the presence or absence of flowers on predation outcome, which suggests that flowers do not increase the conspicuousness of the nest for predators. However, there was an effect of nest concealment. Nests more exposed on the sides and below were depredated more than nests with greater concealment. Aviary data showed Diamond Firetails had a clear preference for longer stems over shorter stems for nest-building, irrespective of the presence of flowers. We conclude that the flowers observed at nests were a by-product of the preference for stem-length for nest-building. This finding helps to explain the inconsistency among anecdotal studies reporting the presence of flowers at nests of Diamond Firetails across Australia.