Rhonda of Northern California, see the blog entry of May 3, took these goslings out and introduced them to the adult geese on the floating platform in her pond, and within 90 minutes they were adopted. Watch her video at http://www.youtube.com/watch?v=qDMz1HiWLJ0.
It works better with geese than other species. With chickens and ducks, it's better to let the hen go broody on eggs and introduce the chicks or ducklings at night. In the morning, the hen will most likely accept them as if they had hatched under her.
If you don't have a broody hen, the transition may not work. Watch closely if you introduce chicks to an existing flock. One flock owner last week recounted how she had separated the new chicks from the mature hens with chicken wire, to give them a chance to get to know each other before she put them together. One of the babies was able to get under the chicken wire and the hens killed it. She plans to hold off until the chicks are big enough to hold their own before putting them in with the flock. She's also increasing the size of her chicken yard and coop.
As with all new additions to the flock, quarantine the newcomers for two weeks to avoid introducing pathogens along with them to the flock.