Background
The ducklings spend just one night in the nest box. In about 24 hours they go from being wet little blobs that can't even lift their heads to active, jumping, and climbing ducklings. It looks like they grow significantly during their first 24 hours in the nest box ... and they do! ... even though there's nothing to eat. Their bodies contain a sac of egg yolk (i.e. bird food) that gives them nourishment for 2 or 3 days before it's gone.
Chickens, geese, and ducks are called "precocial" because their young are ready to go within only hours after hatching. Robins, eagles, and hummingbirds are not precocial, they require many days of parental feeding and growing before they can do anything. A mother duck does not feed her ducklings so they have to leave the nest box to eat, on the day after they hatch.
When the leave, they do it by jumping from the nest box opening. So we call this "Jump Day".
What to Expect on Jump Day
Breakfast break: In the morning after they hatch the mother duck will typically leave the ducklings and go off alone to get some breakfast for a half hour or hour. When she returns they all settle in to rest for a while.
Practice and rest: The ducklings alternate between lots of practice jumping and climbing around in the nest box, and resting. This can go on for many hours in the morning. The mother duck has a sense of when the ducklings are strong and active enough to leave and will not initiate that until they're ready.
Checking for predators: When the duck decides it might be time to go she will jump up into the doorway and stay there watching for a long time, typically many minutes. She’s looking for anything that might harm the ducklings.
Repeat ... and repeat: Usually the first time she looks she will decide it’s not quite the right time and will drop back into the nest box to rest again. Once in a while she will go after the first look-around, but typically she will repeat this multiple times … while we wait.
Calling the ducklings: Eventually she will decide it’s time to go. The earliest this has occurred here is 7:00 AM CDT, the latest was 2:00 PM, and typically it’s around 8:00 or 9:00. She’ll drop down below the nest box and start a special pulsing call that activates the ducklings.
Ducklings respond: The ducklings peep loudly and jump and climb up to the door, and jump out. In a hollow tree nest the opening could be 80 feet (8 stories) above the ground. But the ducklings are very fluffy, and they spread out their feet and flap their tiny wings; no matter how far they fall they land without getting hurt. Usually the time from starting to an empty nest box is only minutes.
Done, and gone: The mother duck keeps calling as long as she still hears peeping from the nest. Finally she’ll lead her ducklings down to the water and they swim off. They never return to the nest; they stay there for only their first night.
After that: A duck does not feed her ducklings; she takes them to where food can be found and they feed themselves. She shelters them at night hiding in tall grass or brush near the water. We seldom see them after jump day, partly because the they tend to go downstream. A mother duck will stay with her ducklings for about 2 months.