Use location services when searching for birds
When searching for birds using location services, the results could be cut down to species that are seen in the relative location of the user. This would eliminate any results that are irrelevant in the user's location
You can search for birds found in the current area using the search box in the Near Me tab. If you are talking about using location to filter from the Birds tab than that would require having a database of checklists for all the areas in the world to know what birds could be found in those areas. I'm actually looking at doing something like this but it's non-trivial.