The closest living relatives of the Terror Birds are the (much smaller) seriemas, which live in the southernmost regions of South America.
The seriemas are the only surviving members of the ancient family Cariamidae which dates back to nearly 63 million years, and which comprised the Terror Birds.
They can be found in northern and central Argentina, though not in Patagonia.
There are only two species, the Red Legged Seriema (Cariama cristata) and the Black Legged Seriema (Chunga burmeisteri). Both measure about 75 to 90 cm tall (30 to 35 inches. Omnivorous, they also eat snakes, frogs, small mammals and insects.
The Black Legged Seriema runs quickly (60 km/h - 37 mph) and only flies if forced to, and then, only for a short distance.
Their monstrous relatives
Imagine their prehistoric relatives, monsters that weighed up to 40 kg (88 lb.) and were about 1,40 m tall (5 ft.), who eat large sized mammals competing with the Sabre tooth cat!.
A recent paper  indicates that one of these terror birds, the Andalgalornis steulletiits "bite's strength" was smaller than expected and that it must have it "applied multiple well-targeted strikes in a repetitive attack-and-retreat strategy" or restrained its prey -though it lacked claws. Its big but hollow beak could not withstand lateral shaking so the bird pecked with repeated boxer-like blows at its victim (see an image here).
 Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled
 Degrange FJ, Tambussi CP, Moreno K, Witmer LM, Wroe S, (2010). Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae). PLoS ONE 5(8): e11856. doi:10.1371/journal.pone.0011856
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©