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.