Until a recent publication by Sarasola & Bechard (J Ornithol. 152:473–479, 2011) it was not confirmed that this common small caracara is sexually dimorphic. These are two different birds, presumably a pair. The top bird is a male showing bright yellow legs, feet and cere while the gray legs and toes of the bottom bird suggest it is a female This is the paler northern race. A darker race is found in extreme southern South America. SACC Proposal 561 to move this species to the genus Phalcoboenus did not pass. Canon PowerShot SX50 HS.
References:
Bierregaard, R.O., Jr, Kirwan, G.M. & Boesman, P. (2018). Chimango Caracara (Phalcoboenus chimango). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/53195 on 28 April 2018).
Chimango Caracara (Milvago chimango), In Neotropical Birds Online (, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. retrieved from Neotropical Birds Online: https://neotropical.birds.cornell.edu/Species-Account/nb/species/chicar1
Fuchs, J., Johnson, J.A. & Mindell, D.P. (2012) Molecular systematics of the caracaras and allies (Falconidae: Polyborinae) inferred from mitochondrial and nuclear sequence data. Ibis 154(3): 520-532.
Remsen, J. V., Jr., J. I. Areta, C. D. Cadena, S. Claramunt, A. Jaramillo, J. F. Pacheco, J. Pérez-Emán, M. B. Robbins, F. G. Stiles, D. F. Stotz, and K. J. Zimmer. Version 28 April 28 2018. A classification of the bird species of South America. American Ornithologists' Union. http://www.museum.lsu.edu/~Remsen/SACCBaseline.htm
Sarasola, J.H. & Bechard, M.J. (2011). Not as similar as thought: sexual dichromatism in Chimango Caracaras is expressed in the exposed skin but not in the plumage. J Ornithol 152:473–479.