The kingfishers were traditionally treated as one family, Alcedinidae with three subfamilies, but following the 1990s revolution in bird taxonomy, the three former subfamilies are now usually elevated to familial level. That move was supported by chromosome and DNA-DNA hybridisation studies, but challenged on the grounds that all three groups are monophyletic with respect to the other Coraciiformes. This leads to them being grouped as the suborder Alcedines.
The tree kingfishers have been previously given the familial name Dacelonidae but Halcyonidae has priority. This group derives from a very ancient divergence from the ancestral stock. Even tropical South America has only five species plus wintering Belted Kingfisher. In comparison, the tiny African country of The Gambia has eight resident species in its 120 by 20 mi. (192 by 32 km) area.
The six species occurring in the Americas are four closely related green kingfishers in the genus Chloroceryle and two large crested kingfishers in the genus Megaceryle, suggesting that the sparse representation in the western hemisphere evolved from just one or two original colonising species.
The smallest species of kingfisher is the African Dwarf Kingfisher (Ispidina lecontei), which averages at 10.4 g and 10 cm (4 inches). The largest overall is the Giant Kingfisher (Megaceryle maxima), at an average of 355 g (13.5 oz) and 45 cm (18 inches). However, the familiar Australian kingfisher known as the Laughing Kookaburra (Dacelo novaeguineae) may be the heaviest species, since large individuals exceeding 450 g (1 lb) are not rare.
They are able to see well both in air and under water while swimming. Their eyes also have evolved an egg-shaped lens able to focus in the two different environments.
The Old World tropics and Australasia are the core area for this group. Europe and North America north of Mexico are very poorly represented with only one common kingfisher (Common Kingfisher and Belted Kingfisher respectively), and a couple of uncommon or very local species each: (Ringed Kingfisher and Green Kingfisher in the southwest USA, Pied Kingfisher and White-breasted Kingfisher in SE Europe).
The Pacific Islands Conservation Research Association is involved with projects studying coral reef fish and endangered Micronesian Kingfishers (Todiramphus cinnamominus) in the Federated States of Micronesia. Additionally the Ornithological Society of Polynesia, studied critically endangered Niau Kingfishers (Todiramphus gambieri niauensis) in French Polynesia with PICRA support.