This small shorebird is restricted to high alpine bogs in the Central Andes. Although the bill suggests a sandpiper this species is currently classified as a plover and sometimes goes under the name "Diademed Plover." It is the only species in the genus Phegornis. These are immatures (bottom three photos) which seems to be transitioning from juvenal to adult plumage. Typical adults have black face and crown year-round while immatures have a more blended head pattern. Top photo may be an adult. An uncommon species, it is estimated to have a world population of less than 7,000 and is declining. It is listed as near-threatened by IUCN. Digiscoped with Panasonic DMC-LX5 | Nikon FieldScope III | 30XWA | hand-held (no adapter).
References:
BirdLife International. 2016. Phegornis mitchellii. The IUCN Red List of Threatened Species 2016: e.T22693931A93430667. http://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T22693931A93430667.en. Downloaded on 13 April 2018.
Diademed Sandpiper-Plover (Phegornis mitchellii), 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/diaplo1
Fjeldsa, J. and Krabbe, N. (1990) Birds of the High Andes. Zoological Mus. University of Copenhagen & Svendborg.
Wiersma, P., Kirwan, G.M. & de Juana, E. (2018). Diademed Plover (Phegornis mitchellii). 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/53857 on 13 April 2018).