dying ducklings please help

My bard rock hen went broody, hatched out and raised 8 chick. So then I was given 9 (10 to 12 days old) Cayugas duckling about a 5 or 6 weeks ago I kept the ducklings in a large acrylic tack for about a week but then decided they and the chickens looked like they needed a bigger space to live. So I moved the duckling (now slightly bigger then the chicks even though they were younger) chicks and mama hen to one of my rabbit runs that was 3 foot wide and 6 foot long with a heat lamp at one end. this worked great and mama chicken didn't even seam to care that her chicks had doubled in numbers and the chicks were not attacking the ducklings either. Then about 3 weeks ago I got 15 mix breed duckling that were about 2 weeks old they were just a hair larger than my duckling and every one got along great and mommy bird let any one the could get under her nap under her and so it would be this mother hen with about a dozen chick under her and another 8 to 10 around her (really wish I had thought to get pics of that) it was adorable. They all lived in a brooder together for about 2 weeks, I removed the heat from the tractor when they were about 4 to 5 weeks old, different hatch dates but all with in 7 to 10 days of each other. A week latter I moved them to a large chicken coop (6 foot wide 10 foot long)when they no longer needed heat I guess every one was between 5 and 6 weeks old but had been out side most there life with heat. Unfortunately after about a week 2 or 3 of the chicks figured out how to get out of the coop So I put them back and thought I had found where they got out and plugged it up but the next day all the chicks and ducklings came running up to me when I went out to feed them in the morning. So I put them back in and tried to figure out where they got out from then less than 4 hours latter they ran up to me again so I decide to move them to the rabbit area which is made with heavy boards, chain link fencing and chicken wire surrounde d by concrete slabs. I was eventually planing on using this area for a rabbit colony so it needed to keep baby rabbits in But it does not have any rabbit loss in it right now. This area is about 30 foot by 30 foot. I didn't loos any duckling tell I moved them to this area. Now I have lost like 5 in a week, 3 in just the last 2 days.

For the first 4 weeks I feed pureana none medicated chick starter I went through 150lb bags in a month. Then some one told me that the Purean could case them to grow to quickly so I started adding the grain mix I give my free range birds (barley and wheat)they seamed to like this and were doing well. they were on the mix for about another 2 weeks when I ran out of the Pureana and decided to just give them the wheat and barley grain, they continued to do well on this tell about a week ago. once moved to the rabbit area they also had acses to some Excel rabbit pellets from when the rabbits spill as well as bugs. and the rabbit area had about 1 bail of straw as bedding to keep the mud under control. I could look it up but I think they are about 8 weeks old now The chicks are fully feathered But for some reason the ducklings are not fully feathered yet they have feathers on the stomach chest and front of the neck but not there backs.

Also I had to move my 4 adult ducks in to the rabbit area last week after they kept finding there way in the the land lords raspberry field and pulling down the canes to eat the raspberries. The oldest hen keeps attacking the duckling grabbing at there back sides and not letting go. Out of the ducklings I have lost it seams random not just the small or large. I have lost 2 of the Cayugas and 3 of the mixed ducklings. so could there be some thing they are eating that is killing them or is the hen attacking them stressing them so much that its killing them. Oh ya for got to mention I was given a bunch of wheat breed for the birds on yesterday So i gave them a loaf on Saturday a loaf on Sunday and then a loaf on Monday. They have to share the breed with the chickens (they jump the fence) and the 4 ducks but some how they still look like they get a lot of the wheat bred. The previous owner of the mixed breed ducks feeds bread twice a day, every day.
sorry for how long this is but i didn't want to leave any info out encase it was important my duckies are so funny i don't want to lose any more if I can avoid it.
Thanks for any help.