Do fake owls deter hawks? I've heard of people trying reflective tape and also hanging blank CD's on string where they can swing around to catch the light.
I suspected a bird of prey, but assumed it happened at night. Perhaps a call into your local wildlife department is in order? Sorry for your losses. Is it possible to net your free range area?
Rats would have to be very hungry and the duck would have to be some how hurt, possibly with open wounds for the rats to go after it. They would rather gather their food (duck feed) than hunt for it!
Can you describe the bite marks? I know this sounds gross, but did you get a picture of the damage? You can normally tell if a rat was the culprit by teeth marks. Rats CAN take down an adult duck (especially if it's more than one of them) but I highly doubt it unless there was something...
Was there snow ON her? If it didn't snow after the incident, it was likely a bird of prey...I'm really thinking owl. Did you look for wing sign or just footprints?