Just joined this thread, how interesting, Pecan, about the egg. Here's my take, though. The duck laid an unfertlized egg, was not ready to be on a "roll", so that one can be thrown away. If you don't get rid of it (take it to the edge of the woods far away, let some other animal have a nice snack) then it may break and be very stinky, as it's likely rotten.
As far as the ducks remaining in your garden, I suspect they have found another more secluded place to begin their nest. Look VERY carefully, as they will hide them well, with both feathers and downy feathers and grasses. They will not lay on them every night, or our ducks never did. They came to the nest, laid an egg a day, then when they had a full nest, or "clutch", then they laid on them. Our ducks would get off the nest to run to the pond, go for a swim, get a drink, them right back on the eggs. Had a mallard female once that laid 24 eggs, 2 layers of them! And she hatched each and every one of them.
Killdeer most often lay their eggs on the edge of the road, or in a driveway. If you have ever seen their eggs, you will know why. They are VERY camofloged in the rocks, cannot pick them out unless pointed out to you. They will also lay eggs in farm fields, but then they get them run over with farm equipment, as it's the same time that the fields are being planted and tilled. They are also the birds that once they start sitting on their clutch, they will run and act like they have a broken wing if threatened, to try to steer predators away from the nest. Baby killdeer look exactly like their parents when they are born, except for a shorter tail. ADORABLE!!! They run around really fast, too.
As with all birds, if they sat on their nests all the time after they laid one egg, and laid maybe 6-10 total, the first egg would hatch a day earlier than the 2nd, and the 2nd a day earlier than the 3rd, etc. So, that's why they don't start incubating them until they are done laying them. Our domestic ducks would lay eggs in the edge of the water before they were ready to be breeding. Not positive, but I don't think ducks have only one mate, but maybe wild ducks do. Our domestic ducks would have a hay day with the females. They were always outnumbered, and it was scary for my kids to watch, thinking the males were going to drown the females, as they took turns with her. LOL Poor girls. We soon learned how to tell the males from females and got rid of most of the males for the following year.
And, with wild ducks, you can tell which is male and female, as the males are prettier. But, with say the all white Pekin ducks, the male will have a tail feather at the base of his tail on top of his back that curls almost all the way around.
With the cow birds, my son hates them! I think that starlings do the same thing, laying their eggs in robin nests. Again, they sneak in before the female starts sitting, and not only lets the other bird incubate them, but also feed and raise them! It's quite funny seeing a larger black bird in a robin nest, but irritating that they have to raise the other babies.
As far as imprinting with the baby ducks, yes, if you are the first ones they see, they will think you are their Mommy and follow you everywhere! Be careful of that. LOL
OK, off my nature lesson.
