Feed duck feed - Buy duck pellets from Agway or another feed store and mash them for very young birds. If the bird is older, you can feed them whole. In both cases add hot water to let the pellets soften. Add shredded lettuce, baby spinach or spring salad.
Rip up the greens so that they are manageable for the baby. The younger the baby, the finer the pieces have to be. You can rip up the greens or use a pair of scissors to cut fine slivers.
Be sure that everything is chopped finely for very young, small goslings or ducklings. If they are bigger, they can rip up greens on their own.
Duck vitamins may be sprinkled on the greens if you can get them. Vionate is one brand.
If you're near a pond, duckweed can be skimmed from the surface of water along with microscopic plant life that ducks love. It's a natural food.
Add the duckweed to the mixture, along with some of the water from the pond, and the ducks and geese will take over from there, as you can see below. The seagull is watching this gosling "pig-out."