Scratch, bread, many other things they LOVE are very high in carbs and cause fat birds. Fatty liver disease is more common than people realize. Anything other than regular feed has to be considered a treat and only given in moderation. No more than 10% of the diet. No one can say if that is what happened here without a necropsy, but diet is one of the biggest factors in a healthy flock. There are loads of things that can make birds sick or kill them and symptoms can be similar for many of them. The absolute only way to know for sure is through necropsy, either a professional one, or an informal home one that can find obvious things. When viral or bacterial is suspected then a professional necropsy with lab work is the way to know for sure.