Jane, I would get in touch with a corgi club in your area. I'm fortunate enough to be close to the Lakeshore Pembroke Welsh Corgi Club (Chicagoland area). Mainly their organization is formed to improve the breed. They have annual dog shows, where last year they offered the herding instinct test to be given over one of their show weekends. We took our fluff Gizmo to see how he would do. I'm hearing more and more of these type groups sponsoring this test, so I would look into it for your area. If you need help finding a group in your area, let me know, I'll help you find some information.
One of the things they mention during the testing is how they judge the agressiveness to the sheep (we were using sheep, but ducks can be used). I watched several corgis being testing, and I only noticed one nipping at the heels, but the sheep kinda kicked at the dog, and it started keeping more of a distance. They learn themselves if they are getting too close. Also we were told a dog will not pass the test if they injure a sheep, or run it into a fence. The whole experience for us and Gizmo was wonderful! I recommend it to everyone who has the opportunity to try it. He did pass his test, it's so cool seeing that instinct kick in for them while the handler works with them.
My Gizmo is a nudger (pokes with his nose), he nudges me when he needs something, and he nudges the cats to get them to run.
I have two cats in the house, one will run if he chases her, and he loves to chase her! She will just hiss, run and hide. He has never bit her, he just wants her to run. My other cat has figured out his game, and will just lay down and refuse to run, Gizmo then just walks away. Funny animals!
I haven't been able to pursue training Gizmo for herding trials, I really don't have any place to practice that would be convienent for us.
Like I said, look into getting Duchess tested first, she may be great at it and you might meet local folks who could help you pursue her ability.