Original owner?? That's absurd. It's all already been said, but you owe nothing to these people and it's obvious they have no clue who the ducks 'belong' to.
In our area, if you take in a stray animal, you must surrender the animal if the "owner" shows up. However, you are within your legal rights--and the rescue organizations all strongly suggest--to charge a daily fee for the care plus any expenses. So, a charge for any feed they've eaten, treats you've given them, water they've used, plus a daily fee for your efforts in caring for them. You are NOT obligated to surrender the animal unless the fee is paid.
People are so stupid and weird. I recommend you contact a rescue organization--even a dog or cat rescue will do--and ask them what the laws are in your area. The animal control department can tell you also.
Then line up your documentation (i.e., write down the dates of each event and a description of where the ducks were found--emphasizing that they were STRAYING off property), when you were contacted and by whom, and refuse to surrender them unless they jump through your hoops. Which they won't.
I wish I had known all of that when we rescued a stray dog a couple years ago. The poor thing ended up back at the "original owner's" house until he strayed again and ended up at animal control. They said they were going to collect him but they never did--they wouldn't even pay the $80 fee to collect their own dog from the pound! Lucky for him, he was adopted very quickly. Unlucky for me, it was by someone else because despite the $250 I had put into him in the week I had him, they would not notify me when he was available. Long story. Sorry.
Anyway, you almost certainly DO have legal ground to stand on here, but you need to find out what it is and then stand there firmly. Good luck.