You must feed a newborn squirrel Esbilac Powder Milk Replacer with added heavy whipping cream. Though this formula is made for puppies, its nutritional profile is also good for newborn squirrels.
Before feeding, make sure the squirrel is warm and hydrated. Use a 1 to 3 cc syringe for feeding, not a bottle. Bottles can allow water and formula to enter the squirrel's lungs and cause infection. Mix the formula with warm water and heavy whipping cream and bring it to room temperature. A newborn squirrel should be fed 1 to 3 cc of formula every 2 to 3 hours, depending on its age. When the squirrel is between 7 and 9 weeks old, you can begin to introduce fruits, vegetables and seeds into its diet.
It is absolutely necessary for squirrels to have diets comprised mostly of calcium early in their lives. Otherwise, baby squirrels will develop a bone disorder and likely die.