The Common Eider has at least three sub-species when comparing these northern waterfowl to each other in North America. These hardy waterfowl are seen from the northeastern US states on the Atlantic coast, right across the arctic from northern Quebec and the northern territories.
They are also seen along all the coastlines of Alaska as well as along the coast of the northwestern Pacific Ocean. These three sub-species would be the Common Eider (eastern), the Common Eider seen in the Hudson Bay and the Common Eider (western).
Every winter when the Arctic Ocean freezes over, the eiders are forced to seek out the southern open seas. Large rafts of these eiders can be seen in the Atlantic and Pacific Ocean.
Avibase - the world bird database This site provides the user with a complete list of bird species, broken down per country, or in the example of the US or Canada, per state and province. Here, bird species names are available in other languages, a great asset to be used as a translation of foreign bird names.
ABA - American Birding Association This site represents an organization that maintains official records of all birds species that have been proven to have been seen inside the perimeters of the North American Continent and the surrounding bodies of water. Regular revised versions are posted to keep the bird list current at all times. This is the list used by all serious birders over their lifetime. You may be aware of the movie called the "Big Year". It was with this list that all the competing birders used in an attempt to set a new record as to how many bird species that could be seen by an individual birder in one calendar year.
I hope you will take advantage of these suggested websites. I have used each of them, in one way or another, throughout the years in my quest to better identify and understand our fine feathered friends.