The image of the male White-rumped Shama (Copsychus malabaricus) by Jason Cho clearly shows the state of two rectrices or tail feathers. The outer right rectrix (on your left in the picture) is a new feather. Note the continuous outline of the vanes except for the distinct fault line just before the tip. This fault line is created during the growth of the feather when the bird was not getting enough nutrients, as feather growth is a energy demanding. On the other hand, the left outer rectrix (on your right in the picture) is an old and worn out feather waiting to be shed. Note the frayed outline and the worn out end portion, a result of wear and tear of this tail feather.
As feathers are dead structures and cannot be repaired when worn out or broken, new ones need to be produced. As a new feather grows out, it pushes the old one from its follicle. This is known as moulting. With tail feathers, partial moulting is seen as one or more rectrices are replaced at a time. It would not be to the bird’s advantage to have a complete moult as otherwise its maneuverability would be compromised.
This post is a cooperative effort between NaturePixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.