You have a leucistic Chickadee. Since I don't know how to properly explain it, wikipedia will: "Leucism is a general term for the phenotype resulting from defects in pigment cell differentiation and/or migration from the neural crest to skin, hair or feathers during development" http://en.wikipedia.org/wiki/Leucistic
You won't see any two leucistic birds that look alike. Congratulations on the neat capture!