It was a striking bird. Leucism is an abnormal
plumage mutation where a random number or all feathers are white, lacking pigmentation.
It is an inherited disorder where the melanin pigments aren't
deposited in the feather cells.The melanin pigment, phaeomelanin, is
responsible for red-brown colour in this American Robin. Link to:
Hein van Grouw, 2006. Not every white bird is an albino: sense and
nonsence about colour aberrations in birds.