Oh, you are so kind to help the poor pekins. What a shame that people are seeing them there and assuming they can just dump their ducks too. The domestic mallards who are larger than the wild ones, are as you suspected, domestic ducks also. They do not have the ability or instinct to migrate.
If you feed bread, please consider whole grain or keeping the amounts to a small bit. Probably most of the other people feed them trash like bread and crackers so you may be the only one interested in providing healthy foods. If you have a feed store nearby, you could also get scratch grains so that they get more than corn. Corn, however, will help them make it through the winter as it is fattening which helps them stay warmer. It's less good for them in the summer. If it was possible to bring some food bowls or tubs to put it in, it would be good for them to have waterfowl feed, gamebird feed, turkey feed, or even chicken feed. However, you may have to get them used to it first. If I get a duck who is not used to real food, they make the transition more easily if you put something they like in with the feed - so some of the corn or scratch grains. But, I understand that using bowls may not work and you wouldn't want to put the real feed down on snow or it will get wet and could mold. Another food that I'm told is good to feed park ducks is regular cheerios because these are whole grains and low in sugar. Catfish food is also good in that it floats and is high protein - the park ducks are likely to not get much protein (which is why they love the worms - they probably crave the protein). In the winter, they can't get much greenery at all, so shredding up some dark leaf lettuce to throw in the water for them is usually very appreciated.
Obviously, the best thing would be if you could find someone who would be willing to adopt the group and provide a safe home with good food. It would be a chore to catch them, but given that they come running to you, it may be quite do-able. The biggest danger to them is that when the pond freezes, they will not be safe from predators. Dogs, raccoons, or any kind of wild animals there (and you might be quite surprised as the presence of fox or other such things) can walk out on the ice and get them, particularly at night. Of course, they also need water to drink to survive. If the pond has a fountain or aerator and the city keeps it running in winter, they should be okay, but if the pond freezes they really are the proverbial "sitting ducks".
You are helping them very much to stay as healthy as possible - but winter is dangerous for them and if a home could be found, it would be really terrific. You might look through the rescuer listings for your area and call around to see if anyone has ideas. I also sometimes advertise on craigslist (under pets) to try to find homes for ducks here who need a new home. Of course, you have to try to find out as much as you can to try to ensure they are going to be pets rather than dinner and that the home is prepared to keep them safe - but they are in a risky situation now and that's a way to try to improve things for them.
Best wishes to you and the lovely ducks. Thank you so much for caring for them.
-Chick Mom/Marlys in Oklahoma