The Passenger Pigeon or Wild Pigeon (Ectopistes migratorius) was a bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

, now extinct, that existed in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and lived in enormous migratory flocks until the early 20th century. One sighting in 1866 in southern Ontario was described as being 1 mile wide, 300 miles long, and taking 14 hours to pass a single point with number estimates in excess of 3.5 billion birds in the flock. That number, if accurate, would likely represent a large fraction of the entire population at the time.

Some estimate that there were 3 billion to 5 billion Passenger Pigeons in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 when Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

ans arrived in North America.