I spent some time trying to find any links on sites for birding in Bulgaria but couldn't find any. There are quite a few places to visit in the country but there is one with a cottage suitable for birdwatching and wildlife photography in Bulgaria which people in this forum might find relevant for planning their trips. The place is called Suha reka and literature review suggests that there are some 200 bird species in Suha reka. It is fairly understudied so pretty wild. You can see more information about the area on this website here http://naturetravel.eu/.