They definitely cross breed. Like any male in heat, they look for any port in a storm.
Each year a few weeks past Easter, the white Peking ducks show up in our local subdivision ponds. Apparently, that cute little ducking that sissy got for Easter got bigger and started leaving quite a mess on the Berber carpeting, so out goes the duck.
Unfortunately, these ducks get in the gene pool with the wild ducks. The offspring are real cute until they start to become huge terrible looking mallards with wings so small they can't fly.
Nature can be cruel to any living thing, but it is especially cruel to animals that are ill prepared for the seasons. These half breed mallard/pekings are just that. When the ponds freeze solid, the wild ducks take to wing, while the cross bred ducks decide that walking to Louisiana is out of the question, so they just stand on the ice and wait for spring.
I am not sure what happens, but I don't know that any have ever made it through the winter. The dogs, coyotes or birds of prey must get them.
My daughter and I have taken on a pair as a project. The ducks need food, water and shelter from predators to survive. There is some water trickling into the pond all the time, so they can get water to drink. We bring a shoe box of bird seed to them each weekend, but the shelter thing is up to them.