My pen is covered in grape vines. The plants are on the OUTSIDE of the pen but grow next to it and up and over. The ducks will eat any leaves or grapes that grow through their side. Some of the ducks will eat fruit that grows low enough. A few of my ducks do not like grapes (weirdos). In the summer their pen gets great shade. And in winter the leave fall and let the sun through. I recommend grape vine to everyone. You and the animals both benefit. The other year I harvested over 50lbs of grapes from 1 vine.
As for choking, grape seeds are small and wont bother the ducks. They swallow live fish whole. Grape seeds wont even slow them down.
I have 1 concord vine and 1 red seedless. The red seedless grows much faster and puts out way more fruit, but I use both grapes for different things. Concords are best for cooking (jams, jellies, sauces, etc...) and the red seedless are best fresh or frozen whole for snacking.