Ducks dying overnight-PLEASE help urgently!

im not sure whats happening to your ducks but seeing as one has already passed away due to it, you need to call your vet immediately! & if possible get your ducks down there! good luck

if the magpie is still with you in the morning then get her to a vets asap, they could have eaten something that doesn't agree with them or they may have something contagious. However if she passes away then again take her first thing to a vets for an autopsy 

The fact that they have all gone off the lay makes me think it is either stress - they've been frightened by something, or it's something they've eaten. Check the feed - is it a new bag (could be contaminated) or are there plants coming up in your enclosure that may be poisonous? Check their water - any algae or is it fed by a river that may have been polluted? Try to eliminate the obvious first, and if no improvement or if another looks ropey, then a visit to the vets is a must.
I've just looked through my Waterfowl book and there is something called Duck Virus Enteritis and it's the only one that seems to fit what you've said -  the symptoms are off colour one day and dead the next, with the bird suffering brown or green faeces. Muscovies are very prone to it. It is spread through contact with wild ducks and also water contamination.
 It states for treatment - cordon off all suspect water sources and clean and disinfect all equipment. Use plastic containers for water as they can be easily cleaned and put Terramycin in their water for 7 days. It may not be it, but hope it helps.

Let us know how they get on.

I think I was posting the same time as Jamima - I've updated my earlier post.