What is the symbolism of Scout's reference to Arthur Radley's feathery hair in To Kill a Mockingbird?Scout's reference can be found at the end of Chapter 29.
The reference that you mention is commonly thought of as yet another reference to the symbolism deeply ensconced within the title of the book. To understand this symbolism, one must return to Chapter 10 where Miss Maudie speaks to Jem and Scout.
That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
"Your father's right," she said. "Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." (Lee 90)
Before speaking of the symbolism here, we must review the example you mention in your well-formulated question. Arthur Radley has always sparked the fascination of both Jem and Scout; therefore, as readers, we shouldn't be surprised at Scout's attention to detail when she finally gets a really close look at Arthur. Among many other things, Scout says the following:
His hair was dead and thin, almost feathery on the top of his head. (270)
Why "feathery"? Because Arthur Radley is yet another "mockingbird" that it would be a sin to "kill." Except in defense of Jem and Scout, Arthur Radley has never done anything to harm anyone. In fact, if we review his actions in the book, they have all been for the good of his neighbors. In regards to Jem and Scout, the treasures hidden in the tree and the blanket around Scout's shoulders immediately come to mind. By the end of the novel, both Jem and Scout know that to expose Arthur Radley would be to "kill" that symbolic "mockingbird."