When you are in the process of weaning rabbits, this is when they stop drinking their mothers milk and start to eat pellets ( or whatever you are feeding them) and start to drink water. If these were wild bunnies, they would start being independent and also stop drinking their mothers milk.
They may get diarrhea at this stage, as their digestive system is still developing and they don't absorb all of the nutrients which they should be.
When they are 4 weeks old you can begin. The mothers milk will start drying up. Look at the litter and decide which one is the largest (you may already know which one is) and pick this one up. This will be the first bunny that you start with. In 2 days take the second largest and do the same. You continue doing this until they are all finished.
When they are 5 weeks old they should be weaned (or most of them) and capable of eating by themselves. If their is any that seem they aren't doing so well (as in don't seem ready yet) leave this one with the mother for a bit longer.
Just keep in mind that they all won't necessarily live due to some diseases which aren't very nice for them to get such as e coli which humans could easily catch as well. There are some more which can result in death and if you are concerned about it I would suggest that you look into it more.
If you breed lots of bunnies, you may find that some take longer then others. I have found out that this is especially the larger breeds yet sometimes it can be the smaller breeds as well.
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I hope this post helps you to learn much, much more about weaning rabbits! If you have any concerns, comment on this post.