In the early 80 s this was a major concern. Wisconsin lost an estimated two percent of wetlands, and as a result the population of mallards decreased. During this time Mallards went from a number of hunting harvests of 199,600 in 1979 to a hunting harvest of 136,500 in 1983. The numbers have since gone up, though. The largest cause of death for mallards results from hunting.
Hens seem to be more vulnerable to death from natural causes.
The predators of the ducklings and eggs are the crow, skunks, raccoon, opossum, coyote, and fox. It is estimated that along the Mississippi about four percent of mallards die from lead poisoning. The Natural History of Mammals. Nielsen, Scott. 1992. The Voyageur Press.