If they are living out at the pond, they will make nests there and try to hatch eggs. You may think you are missing a duck and then a month later there she is with babies!
Ducks lay in the morning, so if they are in a pen/shed overnight and you don't let them out until after they lay, less...
Whatever kind of ducks you get will be quite happy!
Think about what you want ducks for- decoration/pets only, eggs, foraging/slug and pond clean-up, meat. That will help us with some advice.
Storey's Guide to Raising Ducks is a MUST read. It's by Dave Holderread.
The pond with the island in...