What are they eating?

My ducks love to be out in the grass shoveling through it and finding stuff to eat. My question is, does anyone know what they are eating? They plow through the grass with their beaks, making all kinds of happy sounds. But I have no idea what they could possibly be eating. They get way down under the grass so I know they aren't just eating the grass itself. Does anyone know what it is they are so fond of?

 

I had no idea all of that was in my grass!!!

And yes, mine make those piggy noises too. You sure can tell when they've found something extra yummy!
When I come home from work and they've been rooting through the grass all day, they look like swamp creatures with threads of sloppy old grass hanging from their beaks. I have random buckets of water all of the place for them stop and wash off, but they rarely do. They're like a herd of street sweepers just moving up and down the lawn shoveling through the grass. I just love them.

Thanks for the info. I would have never imagined there would be that many creatures lurking in the grass. But if they make my ducks happy, then I'm okay with it.