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Teratorns are an extinct group of giant predatory birds that are classified in the bird family Teratornithidae. The name “teratorn” means “Wonder Bird,” a name given because of the size, uniqueness, and powerful nature of this group of birds. The first species of teratorn known, Teratornis merriami, was described from the late Pleistocene Rancho La Brea tar pits of Southern California, where the remains of more than 100 individuals of this species have been found. Merriam’s teratorn weighed about 13–14 kg (about 30 lb.) and had a wingspan of about 3.2 m (10.5 ft.). Two other genera of teratorns have been described from California, and other species have been described from Oregon, Cuba, and South America. The largest known flying bird, Argentavis magnificens, is a teratorn that lived about six million years ago in Argentina. This teratorn had a wingspan of 6.5–7.5 m (21–24 ft.), weighed about 72–78 kg (158–172 lb.), and had flight feathers that would have been about 150 cm (59 in.) long.
Teratorns were predatory birds that probably captured small animals, such as lizards, mice, and other birds, and swallowed them whole. We can tell this from the structure of their jaw bones, which is quite different from those bones found in New World vultures (such as the California Condor) or raptors (such as hawks and owls). Teratorns definitely did not feed on the carcasses of large mammals, as is so often portrayed in older literature.  
Teratorns had very short legs and broad bodies, so they could not have run very fast. Although they might have captured prey in flight by swooping close to the ground and using their long bill, it is more likely that they were stalkers that moved very slowly while hunting on the ground. They might also have remained motionless, in which case prey would come within striking distance and be seized by the teratorns using their long, hooked bill. Stalking small animals that were struggling to free themselves from the tar seeps at Rancho La Brea would have brought teratorns into situations where they would have become stuck themselves.
The wing bones of teratorns are typical of those of large soaring birds, so their flight behavior would have been similar to that of vultures and large storks. Soaring is a very easy way for large birds to cover large distances in search of food. Teratorns must have inhabited areas with relatively few trees or large shrubs because their large wings would have made maneuvering in forests impossible. The biggest teratorns are all known from semiarid regions.

PDF File Resources

"The World's Largest Flying Bird" (1980) from Terra, Natural History Museum of Los Angeles County, vol. 19, no. 2, 1980.
Chatterjee, Sankar R., Jack Templin, and Kenneth E. Campbell, Jr. 2007. The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina" (2007). Proceedings of the National Academy of Sciences. 104(30):12398-12403.