I have a section of the yard that I scatter shell, and a section I scatter with grit. It’s basically free choice. I’ve switched to ground up egg shell now since we have plenty of it. We save, then wash and dry out the shells. Then I run them thru the food processor and scatter it in a specific spot. I haven’t had any thin egg problems so far.
1 flock block lasts our 40 waterfowl about a month or more. I’m not likely to put it out much now thru late spring since it’ll just melt in the rain. They also get fresh orchard grass, and alfalfa. The ducks prefer grass over almost everything else. One scoop of corn is put out at night now that the weather is turning.
I’m getting 7 eggs a day at the moment. I was getting 5-6, then I changed the way I fed and it went down to 4-5 a day. Once I switched back, it went up to 6-7 a day.
My birds definitely prefer eating shell, grit, and treats off the ground. Those foraging type behaviors are innate, so I encourage it.
Hmmm?...Interesting management plan with Ducks...

Not something I would do, although if it's working for you then so be it..



Just the way you phrased that Edited to add. 
