As I am trying to reproduce the Banded mice I tend to keep pregnant mice littering alone as opposed to 2 littering together so I can check on who makes the best pups, so as a rule I usually bung in a near ready to breed doe ( 10 weeks ) with the preggers mouse to act as nanny which also then tells me if the nanny would make a good mother herself, she also gains pup experience and is less likely to reject her own litter.
Nannies are removed from the litters after about 20 days to be placed in for breeding themselves.
The odd thing with Banded I have found is they don't seem to like other breeds of mice unless introduced at a young age.
I have had pregnant mice retain their babies for longer than they should but then drop them when I remove the unrelated breed nanny, so it seems they do infact prefer to be with their own kind..........racist???
But then Banded mice often tend to be more aggresive towards other mice in general....moody buggers lol.
Great with people though, never had a problem with them.