We'll start with the original plumage and work our way up to current.
Late October, 2011:
Hatch Year (HY) Yellow-bellied Sapsuckers undergo a much more protracted molt than do HY Red-naped Sapsuckers. Given a choice between Yellow-bellied and Red-naped after September, any individual with juvenile head plumage is most likely a Yellow-bellied (Pyle 1997).
Late November, 2011:
In just one short month, the bird has already developed quite a bit!
Early March, 2012:
Hope that helps anyone trying to distinguish young Yellow-bellied from Red-naped Sapsuckers next winter!
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Mlodinow, Steven G., Jessie H. Barry, and Cameron D. Cox. 2006. Variation in Red-naped and Yellow-bellied Sapsuckers. Birding 38:6 42-51.
Pyle, Peter. 1997. Identification Guide to North American Birds. Part I. Slate Creek Press, Bolinas, CA.