Mice can be fed a commercially prepared complete diet - a rodent mix or hamster diet is acceptable. Pellets are available for mice and are completely balanced, but this is a monotonous diet. Grain and seed based loose mixes provide more interest to the mice and will probably be more readily eaten. However, your mice may just pick out their favorite bits and leave the rest, leading to an inbalanced diet. I prefer to feed a combination of balanced pellets and a mixed diet, but only offer small amounds of the loose mixed diet at a time and make sure it is almost all eaten before offering more.
The basic diet can be supplemented with small quantities of fresh fruits and vegetable (greens, apple, carrot, etc.). Watch for diarrhea though and if a particular food item does cause diarrhea, discontinue feeding it. Sunflower seeds are a favorite treat of many mice (but are quite fatty so should be limited), as well as other seeds like millet spray. Cooked pasta and whole grain bread or crackers can also be given, maybe with a tiny amount of peanut butter as a special treat. Commercial treat sticks make a good treat but only very occasionally as they are often quite sweet. Avoid feeding junk foods such as candy and potato chips, and never feed chocolate as it is toxic to mice and other small pets.
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