why you might need to supplementally feed your kittens:
1. The mother cat's body is not producing enough milk (it's called "agalactia"), so the kittens aren't gaining enough weight. This can occur for several reasons, such as inadequate nourishment during pregnancy, large litter, or because it is the queen's first litter.
2. You have weaklings that are crowded out of the nipples by the other kittens.
warning: Even though the kittens may appear to be nursing, that does not mean they are getting milk. Sometimes kittens that have been seemingly nursing for days will die of starvation.
to determine if your kittens are getting enough milk:
2. Do any of the kittens seem weak/not interested in nursing? Are there obvious runts?
3. Some nipples produce more milk than others. If the queen will allow, squeeze each nipple and determine how much milk is coming out of each one (the ones higher on the chest are typically drier). Unfortunately, kittens develop a preference for a particular nipple in the first few days of life - you can try to switch a kitten to a more productive nipple, but he might return to the original one.
4. Weigh them every day. They should be gaining about 1/3 ounce per day.
If the kittens aren't getting enough nutrition, they may not survive without your help. You will need to supplementally feed them.