A beautiful and seemingly large area of tropical rainforest in north-east Cambodia.
You can get a great experience of Keoseima by just getting to Senmonorom, the main town. This is because to get there from the capital of Phnom Penh, you have to drive along a road through a fair chunk of this pretty and varied forest. This is also a good place to see wildlife. The road is wide enough and not that busy, so you can pull over and do wildlife viewing.
The main reason I was exploring the rainforest via the main road, was that it seemed to be the best way to get good views and photographs of the Black-shanked Doucs.
There were also Pig-tailed Macaque to be seen.
In the morning and afternoon, you should see a good variety of birds along the main road, including Great Hornbill, Streaked Spiderhunter, Blue-bearded Bee-eater, and White-bellied Woodpecker.
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There is another site about half an hours drive out of Senomonorom called Dak Dam. Walking along the edge of the forest is quite productive for birds.
It takes about 7 hours by road to get to Senmonorom from the capital Phnom Penh. A nice place to stay at just on the edge of the town is the Nature Lodge. You can also stay in the reserve at Jahoo Gibbon Camp. There is another place on the main road which is located in it's own rainforested valley which looks nice; although I only visited it during the heat of the day when there were many day trippers there, and I did not stay there overnight.