I had an excellent experience birding in Thattekad with Thattekad Birding Tours. He helped me with transportation from and to the airport, local travel, and finding a place to stay. Very reasonable and customizable.
I was interested in seeing many endemic species, and he modified the tour based on my interest. The guide knew the area very well and took me to multiple habitats. In addition, he pointed out the other flora and fauna endemic to that region, along with the birds.
Very friendly and made and effort to make the visit as comfomrtable as possible. Some of the areas in Thattekad, besides the sanctuary, need a guide to get in.
I would highly recommend Thattekad Birding Tours.