By Laura Howard
Anhimidae comprises two genera (Anhima, Chauna) and three species.
Anhimids are neotropical, with a South American distribution.
Screamers inhabit tropical or subtropical wetlands (swamps, marshes, lagoons, lakes), open savannas and meadows, or flood plains of moist tropical forests.
Eggs are incubated for 42-47 days. Chicks are precocial (nidifugous) and covered with gray-yellow down at birth. Fledging occurs at about eight to ten weeks and independence at approximately 12-14 weeks. Moult is gradual.
Screamers are large birds (76-95 cm; 3000-44000 g; 170 cm wingspan), with an overall appearance that is rather goose-like with a small, chicken-like head. Females may be slightly smaller than males. Plumage is mostly gray or greenish-black, with some white on head, neck or forewing. Juveniles have duller plumage and shorter spurs than adults. Ornamental feathers include tufts on nape and slender "horn" on forehead. Feathers are continuous (lacking feather tracts), Iris color ranges from yellow to orange. The short bill is hooked, with reduced lamellae on the inside of the upper jaw and a desmognathous palate. The legs are thick and the front three toes are long and are connected by shallow webbing at the base. The hallux (hind toe) is long, but not elevated. The ribs lack uncinate processes. A pneumatic dermal layer is present beneath the skin. Screamers have two sharp, long, curved spurs on the broad wings.
Anhimids are herbivorous and feed primarily on the leaves, stems, flowers and roots of aquatic vegetation. Adults may also forage on seeds, leaves and stems of other plants, insects and arthropods.
Anhimids are considered seasonally monogamous. Nest-sites and mates may be maintained for successive years or perhaps for life. Pair bond formation involves vocal duets and mutual preening. Both sexes build the nest and defend nest-site territories. Copulation occurs on land, with the male grasping the female's nape with his bill.
Screamers are solitary nesters. Breeding peaks occur from October through December, but some populations may breed year round. Nests are located on or near water and are constructed out of plant material such as weeds, reeds and sticks. The egg-laying interval is approximately two days and clutch size ranges from two to seven eggs. Eggs are white with pale spots.
The male and female take turns incubating for a period of 42-45 days. Eggs are covered with material when parents are both absent from the nest. Parents appear to brood newly hatched chicks for only a few days. Adults may feed or point out food items for chicks. Fledging occurs at eight to ten weeks and parental care terminates at approximately 12-14 weeks.
Anhimids are sedentary, with post-breeding dispersal of juveniles and non-breeders. Screamers have long, broad wings and soar well. The two long wing-spurs may be used in intraspecific territorial or mating disputes. The long toes may help screamers walk on emergent and submerged aquatic vegetation. Anhimids are seldom seen swimming, but when they do they ride high on the water and do not dive.
Anhimids may flock together outside of the breeding season.
Screamers are noted for their extremely loud vocalizations which are audible to a range of three kilometers. Sexes have similar vocalizations, although males may have a slightly lower pitch. Calls range from a harsh two-part trumpeting to guttural drumming.
Roosting flocks are extremely noisy at dusk and mating duets are loud and long lasting during the breeding season.
Screamer feathers have been collected for perceived medicinal preservative value.
Screamers emit loud vocalizations at the first sign of danger and often foil hunters intent on stalking forest prey.
One species, Northern Screamer (Chauna chavaria) is listed as 'Lower Risk' in the IUCN Red List of Threatened Species. Major threats include habitat degradation due to deforestation, agriculture, wetland conversion, and agrochemical use.
Historically, various hierarchies have suggested that Anhimidae (Palamedea or Anhimea) shares putative evolutionary relationships with anatids (ducks, geese swans), phoenicopterids (flamingos), rallids (rails), ciconiids (storks, herons), cracids (curassows, guans, chachalacas), and casuariids (cassowaries, emus, elephant birds). Current molecular, ethological, and morphological analyses hypothesize anhimids as sister to anatids (ducks, geese, swans), and these two groups taken together form Anseriformes. Anhimids are hypothesized to be the most basal taxon within Anseriformes, and perhaps closely related to Magpie Goose (Anseranas semipalmata).
Anhimid fossil remains from Argentina are dated to the Pleistocene. Eocene fossils from North America may represent an anhimid ancestor. Several Paleocene fossils are considered anseriform ancestors: Presbyornis pervetus from Utah and Mongolia; Telmabates antiquus from Argentina; P. isoni from Marlyand.
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Contributors
Laura Howard (author), Animal Diversity Web Staff.
