My duck killed my other duck

[Gigib]

I had 8 ducks, 7 now currently and I was away for a weekend while my family was looking over my ducks so this isn't feom my perspective. Earlier today my mom said that she was outside feeling my ducks before it happened but when she checked on them again apparently my female duck Alex was nibbling on my other female duck Marty (Madagascar characters) Melman my male duck was also nibbling but I was told it was mostly Alex. She had chunks missing as well as bits on her but and one on her neck I believe? I'm not so sure. My mom and brother put medicine but unfortunately she didn't make it since she was missing large chunks. They were raised together as ducklings so I don't know why she would hurt her. I know it's usually male ducks who attack but it was mostly Alex who attacked her. Is there any reason why she would do this and how would I prevent it? I had recently combined my two flocks together as there's 2 males and each had 6 females. They don't fight but aren't comfortable yet since it's been a month but could it be due to the stress of new ducks. I would like to mention they are a year old so they aren't babies

 

[JacinLarkwell]

If a bird is sick, it is not uncommon for the flock to kill them

 

[HollowOfWisps]

How predator proof is your setup? My guess is that a predator attempted to get Marty initially then failed and what your mom was seeing were the others being curious pulling at her wounds which is not uncommon. If another duck has any visible wound especially if there is blood the other ducks will pick at the wounds.