First of all NEVER give bread to waterfowl. It causes them to fill up on the carbs and not eat the protein and calcium they need to stay alive. Feeding that to your ducks would be a death wish. Anything that was intended for a person to eat is not okay for them either, to many spices and salts. Leftovers are tricky business it's hard to find things that weren't cooked in oil or butter which are not okay for them. Feed them lots of veggies, some people feed them meat which is alright as long as it's in small quantities and with no seasoning or anything that was man made on it. Though if you have laying ducks you can give them the egg shells when you make eggs for yourself just make sure to give it in a separate container and not let any non-layers or males eat it.
My ducks love peas, cauliflower and corn. You can also buy them scratch grain for a special treat but things like that and corn are high in carbs as well and deter them from eating their regular feed but they're still healthy to give them as long as it's small amounts. Mealworms are a popular treat as well.
I hope this was helpful even though I came on a bit strong.
Bread is okay to give as long as you don't give a lot and don't give it that often. Think of it like cookies. Eating a cookie isn't going to kill you but you don't want to be eating cookies every day for dinner. That being said, I've never given my ducks bread. Their favorite treats are peas and mealworms.
My ducks are free runners so i cant really control what they eat. I have a big box full with corn in one place but they dont overfeed them self with corn. And i drop leftovers, all kind of in one spot. I hope they dont eat that much.
Lots of people have raised chickens and ducks in the same fashion for years. However, you may want to consider giving them an all flock/flock raiser type feed instead of just corn. Even a chicken feed would be better. It will give them more of the vitamins and minerals they need.
Also, if your ducks are laying they will need a source for calcium. You'll want to offer them oyster shell or crushed egg shells so they don't develop problems, like being egg bound. Even if you give a layer feed, you'll still want to offer a source of calcium free choice.