Sumatra and Papua are the regions with the greatest numbers of bird species-605 and 647 respectively; Celebes and Nusa Tenggara, with 379 and 398, have the smallest. By focusing on one of these regions it is quite possible to come away from a 2- or 3-week birding holiday feeling that you know the birds well. Conversely, if your goal is to see as many species as possible, the advice is to choose an itinerary the includes sites in two or more of these regions.
Taken from Jepson, P, 1997, Birding Indonesia, Periplus Editions. Singapore