Noting that the last post here about birding guides was 2015, is there any new info about it?
I'm not sure there is a guide that specialises in birding here anymore. Elbert Greer from White Sands Dive Shop used to do it. My suggestion would be to contact one of the country's top birding guides, Roni Martinez, who does an urban bird watch tour on San Pedro every year for the Audubon Society. He'd have the best advice. You can reach him through Belize Bird Conservancy https://belizebirdconservancy.org/