Phorusrhachids, the giant terror birds of South America,
are the closest relatives of the secretary bird of Africa, Sagittarius, in the large reptile tree (LRT, 1869+ taxa). The similar, but more distantly related seriema (genus: Cariama) of South America is closer to flamingos (Phoenicopterus) than to terror birds, despite the geographic proximity and tradition.
Secretary birds must have appeared on the planet
prior to the rifting of South America from Africa in the Early Cretaceous, 100 mya (Fig. 1). Afterwards there was a growing ocean between secretary birds and terror birds, as in toucans and hornbills.
This secretary bird video and its narration is wonderful.
It sheds light on how early birds and bird-like theropds lived and preyed on a menu of Late Cretaceous and Tertiary snakes and mammals. Fascinating stuff, especially when the filmmakers propose a new use for feathers.