Bird counts were made on the islands of Java, Bali, Lombok and Sumbawa during a 16 day study in September 1973 in order to compare bird populations either side of Wallace's Line (running between Bali and Lombok). Evidence was collected which showed that the two westerly islands of Java and Bali share a relatively similar bird fauna, but this differs noticeably from the bird life found more to the east. The conspicuous faunal break which occurs between Bali and Lombok appears to represent a transition from a rich fauna associated with the large continental island of Java, to a somewhat impoverished fauna of the outlying oceanic islands. Marked differences in habitat on the four islands were considered to be primarily important in dictating this change.

Using frequency and abundance estimates for the different bird families it was shown that Java and Bali have a preponderance of Oriental (Asian) birds while Lombok and Sumbawa have more conspicuous Australian elements. However, when the presence or absence of species or families alone are considered all four islands have more Oriental types. It seems that a small range of Australian species including honey eaters, wood swallows, zebra finches and parrots are relatively abundant on Lombok and Sumbawa, and are a sufficiently conspicuous feature of the fauna east of Wallace's Line to impress the visitor that he has entered Australasia.

The bird counts illustrate some of the changes originally described by A. R. Wallace (1860).