If its only to fill them up for a couple of days while you get some bunnies then it doesn't really matter too much, i've given mine chicken backs with no problems along with the leftovers from a roast, dog biscuit, cat biscuit, tinned food and ferret biscuit, basically anything meaty. There is always days where you can't feed full carcase, I can get out and catch something most days but not every day. I remember someone who is really against dry feeding feeding nothing but chicken wings which imo is wrong, in all the time i've kept ferrets i've been told and believe that ferrets do best on fur