Question: Why do some birds have such heavily pigmented feathers on their wings and tails?
Answer: Pigmented feathers are more resistant to wear than unpigmented feathers.
That's why woodpeckers, whose tails receive a lot of wear when used for leverage against tree trunks when pecking, have black tail feathers. It's also why White Ibis and Wood Storks, which are essentially white birds, have black on their wings where they are likely to come into contact with tree branches when landing and taking off.
Wood Stork, near right. Downy Woodpecker, far right.