All of em! Really, practically everything will eat a duckling or eggs.
You can eliminate a lot of predators by placing your boxes on metal
poles. Predrill your boxes and poles it makes installation a heck of a lot easier, always install a predator guard on the bottom. You can make predator guards out of metal or buy some 10 gallon plastic pots
cut a hole in the bottom of the pot, slide it up to the bottom of your box then secure it to the bottom of the box with screws. Snakes go after eggs,and ducklings, so do skunks ,raccoons, foxes, coyotes, domestic cats, wild pigs, blue herons, bass ... you get the picture.
Trees are easy to attach your boxes to but a death sentence for what ever nest in those boxes. I can't say it enough, make sure there are no branches overhanging or within six feet of the box. I have witnessed snakes use branches to get at boxes with predator guards on them. Good luck. Make your boxes where they can be opened , this makes cleaning a lot easier. I started making mine so that I can swivel one side by removing just one screw.