The swallows and martins are birds which are characterised by their adaptation to aerial feeding. "Martin" tends to be used for the squarer-tailed species, and the name "swallow" for the more fork-tailed species; however, there is no scientific distinction between these two groups. The entire family contains around 83 species in 19 genera. Swallows have a cosmopolitan distribution across the world and breed on all the continents except Antarctica. It is believed that this family originated in Africa as hole-nesters; Africa still has the greatest diversity of species. A number of European and North American species are long-distance migrants. A few species of swallow and martin are threatened with extinction by human activities, although other species have benefited from human changes to the environment and live around humans.