There are two types of redpolls seen in North America, with a sub-species in each. The Common Redpoll is the most likely finch to be seen in the group. Its sub-species is known as the Greenland's Common Redpoll and is a larger bird. The other species is the Hoary Redpoll, whiter in appearance and having a much smaller bill. Its sub-species is called the Hornemann's Hoary Redpoll and once again is a larger bird.
Redpolls are some of the hardiest finches found on the continent. These arctic nesting birds are only seen in the southern areas of Canada and the northern regions of the US during the winter months. They usually show up at backyard birdfeeders in fairly large numbers. They have formed the habits of nomads, with no regular pattern or predictability as to where they will be seen when they migrate from the north.
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.