Geographic variation in the American Dipper illustrates neoteny (retention of juvenile characters into adulthood) as an evolutionary mechanism. Adults from Alaska and most of North America are quite dark especially on the head, but their juvenile plumage that is much paler with less contrast. This species ranges all the way to Panama where adult plumage more closely resembles that of juveniles. Canon PowerShot SX50 HS.
References:
Ormerod, S. & Tyler, S. (2017). American Dipper (Cinclus mexicanus). 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 http://www.hbw.com/node/58089 on 6 July 2017).
Wilson, Mary F. and Hugh E. Kingery.(2011).American Dipper (Cinclus mexicanus), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/amedip