Recent classifications of Australian birds have been limited to lists of "species" which are inadequate as biodiversity indicators. The Directory of Australian Birds: Passerines fills a huge gap in ornithological knowledge by separating out and listing not only 340 species of song-birds but also the 720 distinct regional forms.

Covering about half the national bird fauna, the Directory provides science and the community with baseline information about what bird it is and where it lives in an Australia-wide context. Identity is taken down to the level of distinct regional population. No other compendium on Australian birds does this.