I have an almost full grid, but am puzzled by the unclued entries and the author of the quotation. Is this anything to do with birds, or am I way off track?
I'm in a similar position cloverjo. But I can only find 4 clashes - symmetrically placed on one of the diagonals - and these letters are in 'Darwin', so was thinking the same about birds. Can't find a likely quote yet though!