Not really a lot to go on but being a turkey I would be thinking about blackhead disease. If you have a look at its liver it will a have rings on it - I'm sure an internet search will find pictures of it. It certainly kills turkeys and can make make the droppings yellow. It is caused by a parasite that can be carried by chickens, so if you have had chickens on the same ground as the turkeys within the last 2 years it increases the risk of the disease. If it is blackhead then you may need some treatment for the other turkeys as they will be at risk.