The total Borneo bird list is 664, of which 51 are endemic to Borneo. Zooming closer, 34 species are montane residents while 3 hill residents and the remaining 14 lowland inhabitants. At least, that's what Quentin and Karen Phillipps says in their field guide (I own a signed copy!).
Record may and will change as the climate changes amongst other factors such as loss of habitat, poaching etc. Besides the recent(2010) discovery of a new species of Flowerpecker here, I bet revised version wouldn't be so soon.
Going to the mountain or ranges would mean one has more than 75% chances of seeing the endemics. Easy? one may ask. Well, it never is...(at least depending what's in your wish-list!)
I for one, loves it at the highlands. Always cool and refreshing... You don't need to move a lot to look for shades. You can sit in a spot for a long period of time and who knows an endemic might jump in front of you such as this lovely babbler...
1 day ago