Identifying mark on tail of male eagle

[NewMexiKen]

Phil pointed out an interesting feature on the right outer tail feather of the male (Pride). There is a dark brown mark as shown in his photos. We noticed such a mark in past seasons, so it must be an inborn or genetic trait, as he must have molted his tail feathers at least once over the past two years. Phil's watermark is next to it in the second photo.