On 5 September 2006, Henk Luten en Jan Dunnink trapped an aberrantly
Hein van Grouw from the Dutch Museum of Natural History (Naturalis) in Leiden was consulted and he concluded that this aberration should be called "brown".
It is formed when the melanines which form the dark colours in birds are insufficiently oxidated. This results in a light brown hue on feathers which in a normal bird would be much darker. On top of this, these birds tend to get even lighter in sunlight. On the inside of the wing traces of brown can still be found. The carotenoid pigments, which in Chiffchaff form the greenish and yellow colours, are present as usual, as is clearly demonstrated by this bird.
More about this subject can be found in the article by Hein van Grouw: "Not every white bird is an albino" (Dutch Birding 28 (2): 79-89, 2006)