This is a leucistic house finch. This type of ‘loss of pigmentation’ is different than albinism, which is a complete loss of pigmentation. After digging around a bit more about this topic, I finally found a great paper that explains many different pigmentation changes that can occur in birds.
The specific term to use for my bird is ‘Brown‘.
Brown as aberration is defined as a qualitative reduction of eumelanin. In this mutation, the amount of pigment remains unchanged but the appearance of the eumelanin is changed (cf Kopf 1986). As a result of an inherited incomplete oxi-dation of eumelanin, black feathers will turn dark brown. The phaeomelanin is unaffected. In bird species in which both melanins occur naturally, this mutation is less obvious than inspecies which only have eumelanin. So, a ‘brown’ House Sparrow Passer domesticus (both melanins present) appears less aberrant than a ‘brown’ Carrion Crow (only eumelanin). How-ever, feathers with a qualitative reduction of eumelanin are very sensitive to (sun)light and bleach quickly and strongly. So, old plumage is often almost white because the feathers are bleached by (sun)light. For that reason, this mutation is sometimes hard to distinguish in the field. In the hand, such a bleached ‘brown’ bird can be recognized by, eg, the colour of the down layer under the contour feathers and the colour of those parts of the remiges and rectrices cover-ed by neighbouring feathers (often inner webs).
© Ilex ~ Midwestern Plant Girl