What is the most common bird that ever lived but now extinct?

The Great Auk (Pinguinus impennis) is considered to be the most common bird that ever lived, but is now extinct. It is estimated that their population may have been as high as 5 million pairs during their peak, making them incredibly abundant. These flightless birds were native to the North Atlantic Ocean and surrounding coastlines, particularly in Iceland, Greenland, and Newfoundland.