I have a year old ferret that will not eat anything else but her kibble. The three others love their ferret treats, banana or a bit of cat food, but I cannot find anything that this one likes. She, of course, is a lot thinner than the others but still eats well and is very active so I am hot worried too much about her being sick, I only wish she could eat a varied diet.
It is believed that ferrets imprint on food at a very young age and once that point is reached, they are less accepting of new foods. The period between age 3 weeks and 3 months is called the sensitive period in animals. This period of time is when events lead to long-term effects, for instance, learning to be afraid of red socks. It is also when learning is easier and knowledge is stored as long-term memory.
What this means for you is that your picky eater was probably not offered much of a variety of food items at a young age, which makes your life more challenging. She probably only was offered kibble and doesn’t even recognize the other items you offer as food. Ferret-specific treats or kitten food is similar to kibble and often easier to introduce, but an owner can find it very challenging to get their ferret to eat meat, mice or fruit. (Fruit should not be given much at all to ferrets, which are carnivores).
As long as your ferret is eating a high-quality ferret kibble her nutritional needs are being met, so I wouldn’t worry too much. It is always worth it, however, to do what you can to encourage her to each other foods in case the brand she eats is suddenly unavailable or she gets sick and you need to get her to eat something prescribed.
To vary your ferret’s diet, try a few different food items, such as Ferretone, Hill's Prescription a/d, Oxbow's Carnivore Care or a meat baby food. Drip or mix some of the new food on her regular food, encourage her to lick these items from a spoon or place some in a syringe and squirt a bit in her mouth to get her to try the new food. Be prepared for a few messy first attempts and never force the food down her throat -- simply place some in her mouth so that she has to lick it. Once she has a taste for the new food, you can offer it occasionally. She may never learn to love it, but may tolerate it.