Liz - Sounds like a case of leucism, a situation where a part or parts of a bird's plumage is/are white; but not albinism where all plumage is white and eyes and soft tissues are pink. Years ago, birds such as your White-throat would be referred to as partial albino, but in today's parlance it is leucistic.
From: liz lee <[log in to unmask]>
To: VTBIRD <[log in to unmask]>
Sent: Sat, Dec 13, 2014 5:30 pm
Subject: [VTBIRD] Unusual white throated sparrow
Thursday evening at dusk in the ongoing snow storm, I saw a white
throated sparrow under my feeders digging and scrapping. There was
something odd about his crown, but it was hard to get a good look with
the poor lighting and wet snow. I saw him briefly yesterday on my way
out the door and a better mid afternoon view today. His crown is in
fact completely white. I have been unable to get a picture so far.