I raised cayuga ducklings last year and will be starting my next batch next week.
I handled mine every day. I picked each up and practiced looking at feet/bills/ body.
I also sat on the floor near them while they swam in the bathtub every night and I hand dried them with towels.
I also hand fed them treats after bath time.
Last years ducks will be a year old in May. I can still pick them up when I need to.
I kept food available 24/7 the whole time they were in the brooder, the whole time they were in their teenager pen and now that they're permanently outside.
I knew exactly how much food they were eating, though, because I measured it every morning when i cleaned & refilled feeders.
I had 3 feeding stations plus grit: starter crumble station, oatmeal station and chopped greens/mashed veggies and baby grit.
I mixed probiotics, brewers yeast & a kelp mix together & mixed it with the oatmeal station.
I used the feeding guidelines from Storey's guides to ducks to guestimate how much to feed each morning plus a little extra for spillage/pooping.. because I had one duckling that HAD to stand in the middle of the pan to eat no matter waht it was. She still does that an adult, too.
Last years ducks wouldnt touch Dumor or Manna Pro grit from our local TSC. Lots of people do very well on those so I can only guess that my local TSC isn't storing properly or my ducks are just weirdos :).