If you switched a chicken egg with that of a duck without the hen noticing, what would happen?

I'm answering on behalf of my dad who grew up on a farm.

He grew up farming banty chickens and ducks, (among other farm animals and crops). Banty chickens are excellent and responsible egg layers, and will not give up until their eggs hatch. When they have a “miscarriage”, they just don't stop trying to lay them. Ducks, on the other hand, quickly loose interest in laying their eggs and wander off. In order to sustain the ducks, my dad’s job was to collect duck eggs and place them under banty chickens.

When they hatched, the banty chickens couldn't tell the difference. But the poor ducklings had a hard time understanding “mom”. Banty chickens dart around eating insects; the ducklings’ natural instinct is to follow their mother in a neat line. Now imagine a bunch of ducklings trying to make a neat line behind their difficult to follow, zigzagging mother.

Oh, and then comes drinking from the pond. A banty chicken will lead her chicks to the pond, and everyone takes a few sips and then proceeds darting about after insects. When a duck goes to the pond with her ducklings, they all just wade in and sip water as they swim around. Now when a banty chicken with ducklings leads everyone to the pond for a few sips of water, all the ducklings instinctively wade in and start swimming around. The poor mother, unaware of her children's swimming abilities, then flips out and runs in circles around the pond frantically clucking at them. All the while, the ducklings swim around wondering “What's up with mom?”.