What is the mass of a pigeon tail feather?
I just weighed a pigeon tail feather (~10 cm) long. The mass was 0.05 g. Although all tail feathers are not equal in length (and all pigeons are not equal in size), this is probably a good approximation.

Measuring the drag coefficient is going to be very challenging, because it will vary with the orientation of the oncoming airflow. A feather falling with its broad surface (in this feather ~1.2 cm in chord width) perpendicular to the flow will have a much high drag coefficient than a feather falling with the broad surface parallel to flow.