What happened??

[JacinLarkwell]

Did they have water when they ate. I know water fowl need access to water whenever they eat

 

[Magnolia Ducks]

No one here is going to be able to give you an exact confident diagnosis on their cause of death, too many possibilities, if you are curious and still have their bodies you could send them into your state veterinary necropsy lab, that'd be your best bet. Read this for more details,


https://www.backyardchickens.com/threads/how-to-send-a-bird-for-a-necropsy-pictures.799747/

https://www.metzerfarms.com/PoultryLabs.cfm


Given the sudden cause of death, I'd be more inclined to say it was related to poor management practices or possibly toxin ingestion. Did she by chance give them COLD water?

Cold water was my first thought too.

 

[Ratchnick]

Specifically If you have your ducklings in a hot car then they get home, give the ducks water that's not warm (room temp about) they will drink so much it lowers their body temp and they die.