Wow that is quite a question! You are going by yourself? Borneo (the third largest island in the world) has parts in three countries (Indonesia and Malaysia, also Brunei though generally one doesn't go there for wildlife photography). And depending on what you are trying to see, could spend a considerable amount of time there. I went for the primates and the highlight of my trip was to see a tarsier (one of my hoped for targets). The orangutans and proboscis monkeys are amazing too. Some people go for the cats and there are several quite rare ones (including the clouded leopard) which potentially could take weeks to find. Going up and down the rivers and seeing the various wildlife (including the pygmy elephants) was another highlight of our trip. So a lot depends on your goals. I for sure would bring two camera bodies (in case one failed) and multiple lenses, including a macro for the insects. I would suggest looking at the itineraries of different groups that go there and seeing what most appeals to you. Not sure I would do a really short trip there (a few days), but would suggest trying if at all possible to spend a minimum of a week and possibly two if you can do it. Some photos with the lenses and focal lengths from my trip are in my Flickr album if interested:
Explore this photo album by ricardo00 on Flickr!
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