I've seen this a couple of times with my ladies. First time was two sisters, shared a single nest and the care of their babies. This time, also another pair of sisters, had separate nests but I decided to get all together in one hut (easier that way) one had 4 and the other had 15! I went in to check on all of them this morning and one Mom had none with her and the other had all 19! You know, I find this strange for duck behavior because I've had other Moms peck at other ducklings from another Mother. Could it simply just be that Muscovy Moms who are sisters, tag team the care of the ducklings? I've tried researching it a little and didn't really find much about that so I thought I would share my observations.