Poor ducks

When we are away from home and return after dark, the ducks sometimes elect to spend the night on the pond. The next night, they're usually back in the pen, waiting for us to close the door. No harm done.
Recently though, after we came home late several nights in a row, the ducks refused to return to the pen at all. We tried herding them off the pond, but they were having none of that.
Duckie (1, 2) disappeared. If I'd laid bets on which duck might be picked off first, it certainly wouldn't have been Duckie, the flightiest one in the group.
Then Runt disappeared. Then Bluebill. Dagnabbit, we were going to be stuck with only male Runner ducks left, and wouldn't the female Muscovy just love that, come springtime? And why won't those dumb-as-a-box-of-rocks ducks come back to the safety of the pen at night?! You'd think they'd notice their comrades getting nabbed.
Then Runt reappeared. I don't know where she'd been... she didn't look injured. I am guessing that something chased her, and it took her a while to find her way home again. I held out hope for the other two girls for a few days, but it looks like they're not coming back.
I'm not sure what's getting them... coyotes, bobcats, raccoons, or turtles. Or something else our game camera hasn't caught here yet.
I'm going to try tempting them into the pen in the afternoon, with corn. But they have to be in the vicinity of the back yard to see it, and I can't have fed them too much in the morning, or it won't work. I'm keeping my fingers crossed.