Go on a nature walk looking for pieces a bird might use to make a nest (soft twigs, hay, grass, feathers, etc). Return home to glue them down with white glue into a paper bowl to form your own birdnest.
Use a stuffed bird (or if Easter time you could use a chick) to set in the nest.
During Easter time, stock up on Robin's Egg type candy. Once a day, or every so often, secretly place an egg under the bird. Young children get the biggest kick out of seeing if their bird laid an egg. And of course, they are excited to have the egg to eat :)