It might be helpful if you add recordings of the birds it overlooks to the ebird lists you hopefully make anyway.I am using Merlin in Switzerland and it does seem to be over enthusiastically detecting Marsh Warblers, even though we have some! It is very bad with some water birds such as Great Crested Grebes which are very noisy during the breeding period but ignores them completely! Sometimes it can be surprisingly good, the other day detecting a Bonelli's Warbler's very brief and distant call that l also heard myself.
Mike
Niels