Wing,
It depends on your target birds and birding effort. If you are a twitcher then search on the web and join tailor-made birding tour.
If you just go to Chitwan and sleep inside the reserve, there should be in-house birding guide in every camp inside the reserve. If not planning to stay inside the reserve, the only option is joining jungle tour and bird as much as you can.
If you will hike in the mountain, probably it is difficult to find a birding guide. But you could send a message in the Oriental Bird Club e-message group. Some locals may help you.
For me, I just bird watched while hiking without a guide. It should be ok if you have certain level of birding skill, fit enough and do some homework before the trip.
Good luck,
Captain
