The species list you see (when adding a sighting or suggesting a species) on Birda are dynamically generated based on the location of your sighting. The lists only contain species that have been previously seen at the general location of the sighting you are posting. The lists are also sorted in order of most to least common species for the location.
These location based species lists are based on hundreds of millions of species sightings generated over the last decade so in the majority of circumstances they are a very accurate reflection of the species that occur at any location.
The benefit of these location based species lists is that they simplify and accelerate searching for the correct species and limit the scope for users to misidentify species. In doing so, we also improve the quality of data we provide to the scientific community.