I try to use the same species whenever possible. When I make a chicken batch, I use chicken livers. I use duck livers when cooking with duck.
I don’t feed beef liver to my group because most of them can’t handle it and vomit shortly after eating it. Also, if I put too much liver in, the kitties get sick. So it’s a balancing act for me!
I usually use chicken livers because I feed mostly chicken and can easily find them.