I have a beautiful pond in my yard that I'd like the ducks to be able to swim and play in. But I also need to get them back into their pen (giant 12x16 structure) at dusk or they get picked off by local wildlife. It seems like I can't do both. Two nights ago they refused to come in off the pond, and overnight I lost 2 of my 8. I tried for HOURS to lure them (Cheerios, mealworms) herd them, etc in. Nothing doing. Any suggestions?