Date of this Version
The dabbling ducks grade almost imperceptibly into the pochard group considered here, and whether Marmaronetta and Rhodonessa should be included in one tribe or the other may be open to some question. As here constituted, the tribe includes 16 species of almost world-wide distribution which differ from the preceding group in several minor details. The hind toe has a larger lobe than occurs in the dabbling ducks, and it presumably aids in diving. The feet are also correspondingly larger and are set farther apart and more to the rear than in Anas. One of the most clear-cut differences is in the tracheal structure of the male. Instead of a rounded and entirely osseous tracheal bulla such as occurs in Anas, the bulla is larger, rather angular, and contains several membranceous fenestrae of varying sizes. Although the bulla structure of Netta and Aythya is strikingly different from that of the Anas, the genera Marmaronetta and Rhodonessa provide such perfect intermediate stages that the evolutionary development of this structure is very clear. In addition, the tracheal tube of males of nearly all species (Aythya novae-seelandiae and A. collaris are apparently exceptions) varies in diameter and is enlarged toward the middle. Unlike the typical dabbling ducks, none of these species have metallic-colored specula, and metallic coloration is restricted to the head plumage of the males of some species. Although some species (especially Rhodonessa and Netta) frequently feed on the surface, all species dive well and typically do not open their wings when submerging. Most species are predominantly vegetarian, but the scauplike ducks tend to consume a high proportion of animal material. Nests may be built on land, usually near water, or on reed beds over the water surface, and females lack the disruptively marked plumage pattern typical of female dabbling ducks. Downy young tend to lack eye stripes and cheek marks, and also generally have reduced back spotting. Most if not all species become sexually mature during the first year; the scauplike ducks may take slightly longer. All species exhibit sexual dimorphism in plumage and/or soft- art coloration, and nearly all species have distinct eclipse plumages. Although metallic-colored specula are absent, the secondaries have a contrasting white pattern in many species.