The feel of the feather of a sick pigeon is much different from that of a healthy bird. The healthy one's feather has a satin smoothness allied with a seeming 'greasy' feel whereas the sickly pigeon's feather is dry and coarse, appearing to be 'brittle' to the touch. The difference is quite unmistakable.
Taken from The Ailments & Diseases of The Racing Pigeon
by Old Hand