This chapter presents the state of knowledge of the avian diversity recorded in Santacrucian beds (late Early Miocene) with an updated systematic summary of all taxa. Phorusrhacids outnumber seriemas, rheas, and basal falconiforms in diversity and abundance. More fragmentary occurrences are reported of pelecaniforms, anseriforms, gruiforms, and ciconiiforms. Body masses of fossil forms are inferred from the dimensions of their hindlimb bones (i.e. femur, tibiotarsus) based on logarithmic equations previously modeled from living analogs. In some cases, body sizes of the extinct species are also inferred from the relative sizes of other bones of similar extant species. Inferences about diet and foraging strategies are based on the size and shape of the limb elements and structural details of the cranial elements, by analogy with extant birds. The predator niche is represented by falconids, four species of phorusrhacids and a seriemid. Phorusrhacids and seriemids probably lived in open areas because of their cursorial capabilities. However, birds such as the waterfowls, limpkins, spoonbills, and darters indicate the presence of temporarily flooded savannas or permanent water bodies in forested areas. Habitat preferences of extant seriemas, rheas, tinamous, and the falconid Herpetotheres are consistent with Chacoan-like conditions, and they are useful to infer by extrapolation similar Santacrucian paleoenvironments for their extinct analogs. Santacrucian environments were probably characterized by seasonality in temperature and rainfall and the presence of alternating areas of herbaceous vegetation with shrubby or wooded areas.