Taxonomy: Colymbus ruficollis Pallas, 1764, Holland.
Closely related to T. novaehollandiae and †T. rufolavatus, both of which have been considered races of present species. Extensive hybridization with latter, and occasionally with T. pelzelnii; has interbred in SW England with vagrant Podilymbus podiceps, producing hybrid young. Internal taxonomy complicated and poorly understood; full revision required. Race tricolor sometimes considered a separate species (normally including collaris), on grounds of darker underparts and longer and more massive bill, as well as more pronounced sexual dimorphsim in bill length; however, although belly very much darker, bill appears only generally rather larger and no other differences apparent, so distinctiveness insufficient for separate species status. Race albescens has traditionally been included in capensis, but appears distinctive, especially with yellow vs brown iris. Races intergrade in some areas, e.g. albescens with poggei in Myanmar and W Thailand, cotabato perhaps with tricolor in zone from S Philippines to N Moluccas, and tricolor with collaris in New Guinea. Populations from Java and Lesser Sundas sometimes separated as race vulcanorum on basis of larger and more noticeable throat patch, but geographical reliability of this feature appears unclear. Race poggei includes proposed forms japonicus (Honshu) and kunikyonis (Ryukyu Is). Nine subspecies currently recognized.