The screamers are a small clade of birds (Anhimidae). For a long time, they were thought to be most closely related to the Galliformes because of similar bills, but they are instead more closely related to ducks (family Anatidae), most closely to the magpie goose (which some DNA evidence suggests are closer to screamers than to ducks). The clade is exceptional within the living birds in lacking uncinate processes of ribs. The screamers are represented by three species, the horned screamer (Anhima cornuta), the southern screamer or crested screamer (Chauna torquata) and the northern screamer or black-necked screamer (Chauna chavaria). A penis is absent in the males, and the birds' skin has a layer about a quarter of an inch thick that is filled with small bubbles of air, which produce a crackling sound when pressed.