Fostering is less risky than hand-feeding a baby that is less than one and a half weeks old. (About the time they first open their eyes. )
Use only clean bedding – soiled bedding materials can harm the baby mouse.
Be aware that there is a very real risk that the mother may abandon the new mice (or all the babies. ) Be very careful when introducing the baby mouse, and leave mother and babies alone when you don’t need to handle them. You will hear loud squeaks if something bad happens, so there is no need to watch the cage.
If the babies have “milk bellies” (a visible white band or area on their stomach, which is the actual milk in their tummy), they are getting enough milk from the mother without your help. Check the babies several times a day during the first few days, to make sure they have milk bellies and are not losing weight. Babies can lose weight very quickly, and will need to be fed right away.
Moving them carries the risk that the mother will abandon her babies, so weigh the risks very carefully before trying it.
Try to handle them as little as possible, but don’t worry too much about this. Unlike birds, mice will not reject their young simply because they have human scent on them. After 4-6 hours, if you check the babies and they do not have white bands on their tummies (“milk bellies”), this means they have not been fed. The mother is either deceased, or has abandoned the nest.
If there is no rehabilitation office in your area, the next best option is to try to hand-feed the baby. When you call, ask the rehabilitator what they plan to do with the mice. They may try to raise them, or they may use them to feed other animals they are taking care of. If you don’t want your baby mouse to wind up as owl food, you’ll need to care for them yourself.
Always wash your hands after handling wild mice. They can carry a very dangerous, highly contagious virus called Hantavirus. If the baby survives the first few days, transfer it to a glass or plastic ventilated container that it won’t be able to chew through.
Be sure the heating pad isn’t too hot – if it feels even slightly hot to your hand, let it cool in the box for a minute before pressing it up against the mouse. If you don’t have a heating pad, try using a bottle or bag filled with warm, cooked rice. You’ll need to keep warming or refreshing the rice as it cools. Use any kind of bottle that can be reheated – a plastic bottle or thick zip-lock freezer bag would work well. A healthy mouse will be able to maintain its own body temperature at about 2 and ½ weeks old, as long as its container is in a warm, indoor room.
Baby mice begin to grow fur at about 3-5 days old. Baby mice will open their eyes at about 10-14 days old. Soon after a baby mouse opens its eyes, it enters the “flea” or “popcorn” stage. A healthy mouse at this stage will jump around a lot and will be extremely difficult to hold.
Hold the baby upright (vertical) when feeding it – never rest it on its back, like a human baby. If you do see a bubble, immediately flip the baby upside down (tail up, head down) to prevent any more liquid getting into its lungs. Sadly, a young baby mouse is unlikely to survive aspiration. There is a very slight chance that an older baby will survive, if you turn it upside down immediately.
A newborn mouse will need to be fed every 1 to 2 hours if it is to survive, all day and all night. Someone will need to stay up through the night to feed it. When the baby’s eyes open (around 2 weeks old), you can decrease feeding times to every 3 to 4 hours.
Feed your baby kitten formula, watered down to half strength. Baby mice can’t digest the thicker formula. Hold the baby upright when feeding, to avoid getting any formula in its lungs. Grasp the baby around its middle portion (where a human’s waist would be), with the baby’s head at the top and its back feet at the bottom. The front feet will probably be inside your hand, depending on the size of the mouse and the size of your hand. Dispense the kitten formula into the side of the baby mouse’s mouth. Be very careful not to get any milk in the baby’s nose – the mouse will suffocate. You can dab its face with a Q-Tip after each dose of milk, to keep the airways clear. If the baby appears to be losing weight, try to increase the amount of food. Never force the baby to eat – feed it gradually. If the mouse is extremely small, you can use a (new, clean) fine-tip paintbrush instead of a syringe. Dip the brush into the formula, then dab it into the corner of the baby’s mouth.
Be careful not to rub so much that you irritate the baby’s skin. If the baby mouse will not go potty after several minutes, let it rest. Come back half an hour later and try again.
Suitable hard foods include: premixed rodent food (hamster food is best), cooked rice (they usually prefer white over brown rice), human baby food, and kitten food.
Don’t give mice water in a bowl – they can easily drown.
