Why does the distribution of the Honeyeaters (Meliphagidae) conform so well to Wallace's Line?

Honeyeaters are the most diverse family of birds in Australia, and occupy many habitats from Bali, through New Guinea and Australia, to Hawaii. However, only one species has crossed Wallace’s Line. Five hypotheses could explain the failure of honeyeaters to penetrate the Oriental region. 1. Failure to disperse across sea barriers seems unlikely, because honeyeaters have reached many isolated islands in the Pacific. 2. Sunbirds and bulbuls are potential competitors of small and large honeyeaters, however honeyeaters are aggressive and dominate most other birds. 3. Honeyeaters have co-evolved with the Australian flora, notably the Myrtaceae and Proteaceae, which decline west of Wallace’s Line. However, they also occupy rainforest in New Guinea, similar to that in South East Asia, and are flexible and generalised, using fruit and alternative carbohydrates in Australia. 4. Bird-eating raptors occur throughout Wallacea, so they should not prevent the spread of honeyeaters. Also, while Java and Borneo have numerous placental carnivores and primates, there is some geographical overlap between honeyeaters and these potential arboreal nest predators. 5. Avian malaria probably exterminated native birds in Hawaii, including some honeyeaters, but mosquitos must regularly be blown between islands and birds migrate between South East Asia and Wallacea and Australia, both able to carry malaria between these sub-regions. Overall, none of the possible hypotheses explains the nearly total failure of honeyeaters to penetrate west of Wallace’s Line. We are unlikely to be able to explain the distribution of honeyeaters and other groups in this region until we understand better their ecology and behaviour.