Are leeches a big problem in rainforests and birding sites?

No, leeches are not a big problem. Most of birders wear leech socks to prevent their legs from leeches. Applying some insect repellent is also useful. When the ground is wet leeches are more active. Leeches are not found in all birding sites.

But there are leeches in birding sites located in the wet zone and some highland areas of Sri Lanka. Specially Sinharaja rain forest, Kitulgala rain forest, Surrey bird sanctuary, and some areas of highlands such as certain places in Horton plains and Nuwara Eliya.