If for some reason wild baby mice have been placed in your care, I have included a few tips on caring and feeding to help you give them a chance at life. Please be aware that even in the wild, mice have a 50% chance of surviving beyond 5 months of age, given a normal upbringing. The mice that do make it can live up to around 5 years if healthy. A pair will have a better chance of survival than a lone mouse.
As soon as you have your baby mice, it is important to keep them safe and warm. You can use a small pet carrier, large plastic tub or any other suitable box in which you can put them. Layer the bottom with a towel and place the mice on top. Then, use another soft material to lightly cover the mice like fleece. Place the box in a warm place, ensuring that it is not hot; Otherwise the mice will become dehydrated. A heater on the lowest setting may be all that is needed. Test the towel the mice are laying on so that it feels cozy and warm to your hand.
If the baby mice are less than 14 days old they will need dropper fed with a milk substitute until they are weaned. They usually open their eyes when they are just about weaned and able to eat by themselves. You will need to feed them every 2 hours so be prepared to get up during the night. Set your alarm. When I was looking after wild mice, I was getting up every 2 hours to check them. I have since read that during the night mother mouse would be away looking for food and may only return to the nest once to feed her babies. Use common sense, if you can manage a few night feeds, all the best for the babies chance of survival, especially in the early days.
Kitten milk is available to buy at pet shops. I used raw coconut blended up and strained. It should be 1 cup coconut to about 2 and c cups water. You could also use soaked almonds to make an almond milk using the same ratios. Make sure the nuts are natural and plain. Once you've made up the milk, store in a sterilized glass jar and keep in the fridge until required. When you go to feed the mice, take a quarter a cup of the milk and warm it up by pouring it in a small jug and standing in hot water. Use a dropper or baby syringe (you can get these at the chemist) to feed 1 or 2 drops of the milk at a time into the baby mouse mouth. When the mice are really young, they might not open their mouth. Be careful not to get the milk up their noses, they will splutter / cough if you do. It can be dangerous for their health if you do. The way I fed the mice was to put a facecloth down on a table and to put one mouse at a time onto it. Then you can gently hold the baby's head while you manage the milk with the dropper. You'll get the hang of it with a bit of practice. The baby might not seem to be taking much milk, do not worry. Very young babies may only need to get a drop or 2 on their mouth / tongues until they can take more. The main purpose here is to keep them hydrated with a little drop at a time every couple of hours.
Once the baby is fed, you need to stimulate a bowel movement. To do this, put some warm water in a small bowl and dip a cotton bud into it. Then, put the cotton bud between the baby's back legs and gently turn the bud around. You should see a little brown staining, that is their poo. Dip the other end of the bud into the water and gently stroke the baby's body, this emulates the mother licking them. After all, tuck baby up in it's soft bedding and place in a warm spot. This is the basic routine that needs to be repeated every couple of hours during the day and at least 2-3 times during the night, especially around 1am and 5am.
As you can see, it's quite a commitment, looking after baby wild mice. But, there is also a great reward in the caring of them and the bonding you will feel as a carer.
When the babies start to open their eyes or at least take a peak, they may be taking a lot more milk and starting to walk around a little. This is when you need to be extra careful; One fall is enough to be fatal. You can make a little safe roaming area in the bottom part of a pet cage or shoebox / basin. Line it with newspaper and leaves to simulate a natural environment. The youngsters will enjoy stretching their legs and taking their first steps. This is important, as it will build up their muscles and strength.
Once the babies start to bite your fingers quite firmly when you feed them, they may be ready for a little solid food. They will also be starting to open their eyes (12-14 days old) Start very slowly with this. Try some baby fruit puree to start or natural rice pudding. The food should not be chilled. Let them lick it off your finger. Avoid putting the purees onto a dish for the mice to feed from, as they might get messy and end up with matted fur, which needs to be avoided. Some other foods to graduate to are porridge, banana, tomato, dried oat flakes, strawberry. Just go real easy with the food and keep it simple and easily digested to start with. Congratulations! You've actually made it to the weaning stage, which is quite something with wild baby mice.
Continue to provide a safe space for the mice to sleep and once weaned, they will be able to come out at night to feed. Give them a little dish near their bed so they can feed during the night. At least you can get some sleep now! Continue to offer the milk during the day and provide some water for them. Usually, wild mice will still have some mothers milk up to about 4 weeks of age.
Now you have to decide whether to keep them or release into the wild. I do not know how many mice have been successfully hand-reared and released into the wild. I would think it unknowingly that they can survive. However, you've done your bit and if they seem strong and healthy and quite active, it might be possible to release them. Or, you can keep them as pets.
Finally, if you did your best and the mice died, do not feel bad. Survival opportunities in the best of conditions ie with their natural mother are still low. Just enjoy the experience you've had with them and the opportunity to get a glimpse into their little lives. They are little bundles of love and it is wonderful to have at least given them some love when otherwise they might have perished.