Lethargic ducklings dying!

[SoggyForks]

One of my fawn and white runner ducklings is being super lethargic. I had another one acting this way the other day and she died - does anyone know what to do? She doesn't feel very warm so I brought her inside but I don't know what's happening!

 

[SoggyForks]

Update - she's a bit more lively but still very tired. Any advice would be helpful. I have her with some wet foot and water and a heating pad.