A full grown rat snake cannot kill a grown duck. To kill its prey a rat snake has to swallow the prey whole and an adult duck is simply too large for even the largest of rat snakes. My guess is the rat snake was attracted to the area by the scent trails of mice.

And it would have done a heck of a job dispatching them. I watched a few months ago a rat snake, a good five feet long, squeeze through an unbelievebly narrow crack in the foundation to go into my old cellar. I just stood and watched her and let her be. Rather have a rat snake in my cellar than a rat. That's why that snake is called a rat snake.

ok thanks everybody

This summer I watched one dispatch a mole. Not sure if they're as effective as cats, but anything that eats mice and moles is OK by me.

oh I can't stand to see an animal kill another...But I'm glad to know they wont hurt my ducks.

snakes eat alot of slugs too. When I was a kid me and a friend caught about 20 garter snakes and put them in a garbage can as their new home. While we were handling them quite a few barfed up slugs. We played with them for a few days and then let them go, the snakes, not the dead slugs...lol.