The Common Raven, also known as the Northern Raven, is a large, all-black passerine bird. Found across the northern hemisphere, it is the most widely distributed of all corvids. There are at least eight subspecies with little variation in appearance—although recent research has demonstrated significant genetic differences among populations from various regions. It is one of the two largest corvids, alongside the Thick-billed Raven, and is possibly the heaviest passerine bird; at maturity, the Common Raven averages 63 centimetres (25 inches) in length and 1.2 kilograms (2.6 pounds) in weight. Common Ravens can live up to 21 years in the wild. Young birds may travel in flocks but later mate for life, with each mated pair defending a territory.