A vulture is a bird of prey that scavenges on carrion. There are 23 extant species of vulture (including Condors). Old World vultures include 16 living species native to Europe, Africa, and Asia; New World vultures are restricted to North and South America and consist of seven identified species, all belonging to the Cathartidae family. A particular characteristic of many vultures is a bald, unfeathered head. This bare skin is thought to keep the head clean when feeding, and also plays an important role in thermoregulation. Vultures have been observed to hunch their bodies and tuck in their heads in the cold, and open their wings and stretch their necks in the heat. They also urinate on themselves as a means of cooling their bodies. A group of vultures in flight is called a 'kettle', while the term 'committee' refers to a group of vultures resting on the ground or in trees. A group of vultures that are feeding is termed a 'wake'. Taxonomy Although New World vultures and Old World vultures share many resemblances, they are not very closely related. Rather, they share resemblance because of convergent evolution. Early naturalists placed all vultures under one single biological group. Carl Linnaeus had assigned both Old World vultures and New World vultures in a Vultur genus, even including the harpy eagle. Soon anatomists split Old and New World vultures, with New World vultures being placed in a new suborder, Cathartae, later renamed Cathartidae as per the Rules of Nomenclature (from Greek: carthartes, meaning "purifier") by French ornithologist Frédéric de Lafresnaye. The suborder was later recognised as a family, rather than a suborder. In the late 20th century some ornithologists argued that New World vultures are more closely related to storks on the basis of karyotype, morphological, and behavioral data. Thus some authorities placed them in the Ciconiiformes family with storks and herons; Sibley and Monroe (1990) even considered them a subfamily of the storks. This was criticized, and an early DNA sequence study was based on erroneous data and subsequently retracted. There was then an attempt to raise the New World vultures to the rank of an independent order, Cathartiformes not closely associated with either the birds of prey or the storks and herons. Old World vultures The Old World vultures found in Africa, Asia, and Europe belong to the family Accipitridae, which also includes eagles, kites, buzzards, and hawks. Old