Help! My 3.5 week old runner duck may be very sick?

[BuckDucks]

Hi! So I put the grit in their food for two days and then separate it? I think that's what the bag and some guy told me.
Seems like a good plan, don't want them gobbling up too much at first.
It's so weird to me that they eat rocks !

 

[Miss Lydia]

Hi! So I put the grit in their food for two days and then separate it? I think that's what the bag and some guy told me.
Seems like a good plan, don't want them gobbling up too much at first.
It's so weird to me that they eat rocks !

I've always either put the grit into a bowl close by the feed or just sprinkled it on the bottom of the brooder.

They eat very small rocks.