The nature of feather construction
Stages of plumage
A bird passes through various distinct stages of plumage. The plumage of the nestling stage is mostly down and contour feathers which plays a role primarily of warmth and concealment.
Male to female can be quite different, especially in the adult. Some immature birds take on the appearance of a mature female (i.e., some cockatoos and parrots.)
For the fly tier this can be of importance since some feathers in a fly may be obtainable only from an adult male, an adult female, an immature male and/or female, either the adult female or an immature bird, or perhaps any of these.
For example in The Salmon Fly, George Kelson's dressings for "The Silver Spectre" and "Prince's Mixture" call for the use of Black Cockatoo's tail. Experience teaches that the feather of choice is only found on female or immature male Red-Tailed Black Cockatoo mottled orange, black and yellow center tails. A mature male has completely different black-red-black center tails. Then in Francis Francis' A Book on Angling another dressing may simply read "Black Cockatoo or any other black feather." Here the feather becomes more obvious and might be either a strip from the black portion of an adult male Red-Tailed Black Cockatoo's tail, or better yet the all black tail of an adult male Palm Cockatoo.