All individual organisms that scientists have named have scientific names, given by international committees by agreement. Scientific names, Latin or Greek based, are useful internationally. Common names can be confusing across different languages. But it gets even more complex. What is a species? Scientists argue this concept back and forth and often one or a group of organisms keeps getting reclassified.
We used to have the Yellow-shafted Flicker and the Red-shafted Flicker; now they are called the Northern Flicker. We had the Myrtle and Audubon’s Warbler; now they are the Yellow-rumped Warbler. Conversely, we used to have the Plain Titmouse; now we have the Oak and Juniper Titmice. Once juncos were seven species; now they are the Dark-eyed Junco. Split off from the Canada Goose, we now have the Cackling Goose as well. And so on. A committee of professional Ornithologists from the American Ornithologist’s Union makes these decisions, based upon new information, which may come from DNA, song dialects, plumage or behavioral differences, etc. When two species become one, we call it lumping; when one species becomes two, it’s splitting. Of course those birdwatchers who are serious about listing, would rather see splitting (which lengthens their life list) than lumping (which shortens it.)
Look at the Song Sparrow. It has a wide distribution all over North America and its populations are subject to different environmental and thus evolutionary pressures. As a result, at least 20 different subspecies have been identified. All Song Sparrows are Melospiza melodia; but the population from Suisun Bay, CA, is Melospiza melodia maxillaris and the one from the Central Valley, Melospiza melodia heermanni. These subspecies are capable of interbreeding but the supposition is that they will eventually develop different enough characteristics to be their own species. This is argueable. But there is no question that birds have different characteristics in different areas. Birds have darker or lighter coloration, have more or fewer spots, and have different song accents.
Although it is sometimes difficult now, it used to be much harder to delineate a species because ornithologists relied mostly upon physical characteristics. So the more one species resembled another, the more closely they were related, or so it seemed. Then along came DNA analyses. One of the many things this technique uncovered is the relationship between storks, herons, egrets, and their new-found relative, the New World vultures. Yes, vultures are more closely related to storks and herons than they are to hawks and falcons. There are lots of other changes as well. Interestingly, one researcher has speculated that DNA research justifies splitting almost all 10,000 species in two, resulting in 20,000 bird species. That should make listers happy.