poison to ducks

[diablodog0311]

I had 2 ducklings for about 3 weeks and was planning on getting a couple chickens but just today I let the ducks roam around the yard more than I have before and I went to check on them amour an hour later and one was already dead and the other was writhing in pain. So I put them down. But now I'm worried what might have killed them. And I don't want to bring a chicken into the yard now without knowing what killed them. And I'm waiting on a puppy I will be getting next month that I'm worried about. I did use spray weed killer a few months ago but I was thinking maybe it was something from the mesquite tree Or orange tree. Any guess will help to eliminate possible harmful things in my yard.
thanks.