Anseriformes is an order of birds also known as waterfowl that comprises 178 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which includes the other 174 species of waterfowl, among them the ducks, geese, and swans. Most modern species in the order are highly adapted for an aquatic existence at the water surface. With the exception of screamers, males have penises, a trait that has been lost in the Neoaves, the clade consisting of all other modern birds except the galliformes and paleognaths. Due to their aquatic nature, most species are web-footed.
Evolution Anseriformes are one of only two types of modern bird to be confirmed present during the Mesozoic alongside the other dinosaurs, and in fact were among the very few birds to survive their extinction, along with their cousins, the Galliformes. These two groups only occupied two ecological niches during the Mesozoic, living in water and on the ground, while the toothed Enantiornithes were the dominant birds that ruled the trees and air. The asteroid that ended the Mesozoic destroyed all trees as well as animals in the open, a condition that took centuries to recover from, with some models estimating that greenhouse effects lasted for thousands of years. The Anseriformes and Galliformes are thought to have survived in the cover of burrows and water, and not to have needed trees for food and reproduction. The earliest known stem anseriform is the presbyornithid Teviornis from the Nemegt Formation of Mongolia. Some members apparently surviving the KT extinction event, including presbyornithids, thought to be the common ancestors of ducks, geese, swans, and screamers, the last group once thought to be Galliformes, but now genetically confirmed to be closely related to geese. The first known duck fossils start to appear about 34 million years ago. Waterfowl are the best-known examples of sexually antagonistic genital coevolution in vertebrates, causing genital adaptations to coevolve in each sex to advance control over mating and fertilization. Sexually antagonistic coevolution (or SAC) occurs as a consequence of sexual conflict between males and females, resulting in coevolutionary process that reduce fit, or that functions to decrease ease of having sex.
Taxonomy The Anseriformes and the Galliformes (pheasants, etc.) belong to a common group, the Galloanserae. They are the most primitive neognathous birds, and as such they should follow the Palaeognathae (ratites and tinamous) in bird classification systems. Several unusual extinct families of birds like the albatross-like pseudotooth birds and the giant flightless gastornithids and mihirungs have been found to be stem-anseriforms based on common features found in the skull region, beak physiology and pelvic region. The genus Vegavis for a while was found to be the earliest member of the anseriform crown group but a recent 2017 paper has found it to be just outside the crown group in the family Vegaviidae. However, the monophyly of Vegaviidae was questioned by Torres et al. (2025) who described a nearly complete skull of Vegavis in 2025, supporting its placement within crown group Anseriformes. However, Irazoqui et al. (2026) who redescribed this skull suggested that Vegavis can only be confidently placed as a neognath of uncertain affinities. Below is the general consensus (prior to Torres et al. (2025)) of the phylogeny of anseriforms and their stem relatives.
Systematics Anatidae is traditionally divided into subfamilies Anatinae and Anserinae. The systematics, especially regarding placement of some "odd" genera in the dabbling ducks or shelducks, is better resolved following the genetic analysis by Buckner et al. (2018); this has led to the reassignment of many genera to different tribes to which they were traditionally assigned. The list below follows the AviList, which has accepted these revisions: Note that AviList only includes extant and recently extinct genera:
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