last tail feathers to replace

Ducks hatched this summer carry notched tail feathers (circled) compared to non-notched tails. The notch is from the natal down breaking off the shaft of the first set of tail feathers.

Young birds molt those feathers in the fall and grow adult-like pointed feathers (top tail).

Species that use the center feathers to display their quality – like curly mallard feathers or long pintail tails – molt those center tail feathers last. That allows the young bird more time to be in good condition and grow feathers that impress the ladies. 

That is shown in picture 2 of a pintail tail with replaced outer feathers but one really worn center tail feather yet to be molted.