The avifauna of Borneo is of Indo-Malayan affinities. The number of birds endemic to Borneo is relatively small; most species are shared with the Asian mainland. Only 29 birds are known to be endemic to the island and 17 of these are montane. The large proportion of montane endemics is not surprising, because Borneo has been connected with the Asian continent in recent geological time; lowland isolation, and differentiation, has been less extensive than the montane. The Sunda Shelf, on which Borneo is situated, lies in a shallow sea generally less than 300 feet deep. Beaufort has shown that the Malay Peninsula, Sumatra, and Java were connected until early historic times (Darlington, 1957:488).

The endemic species in Borneo are members of four, possibly five, genera that are also endemic. Four of these five genera are montane in distribution. The only endemic for which the geographic history cannot be adequately explained is the monotypic Pityriasis gymnocephala. Its affinities seem to be with the Cracticidae of New Guinea and Australia. The species has been found throughout Borneo. Since Pityriasis is endemic to Borneo, it probably was detached from the parent stock at an early period. The Australasian affinities of Pityriasis emphasize its zoogeographical peculiarities. A more detailed discussion of this species appears in the annotated list below.