I'd first gather a bunch of reference to match your bird.
The BBC has a tonne of great video reference online that
you can make use of:
http://www.bbcmotiongallery.com/
Once you're ready, the next step is to ensure you get your
joints positioned properly. The shoulder, elbow, and wrist
joints are quite different in birds, and differ in position
from bird to bird.
From there, you can rig your wing quite like an arm. Now,
to the feathers which was your main question. I like to use
bones with expressions. That allows me to set rotations
easily that are based on the rotations of the main bones in
the wing. It's actually quite simple to take the wing bone
rotation, distill it down to the proper rotations for your
feather bones, and multiply it by a scale factor that will
either stretch the feathers further, or compress them in
as the wing reaches out and comes close to the body.
Quick example:
//distal
if (rt_wrist_jnt.rotateZ >= 0)
{
rt_wing_spread_01_jnt.rotateX = rt_wrist_jnt.rotateZ * -.30 ;
}
else
{
rt_wing_spread_01_jnt.rotateX = rt_wrist_jnt.rotateZ * -.60 ;
}