Even non-breeding female birds can lay eggs.

Just because a female bird is kept as a single pet doesn't mean that it won't occasionally lay an egg (infertile, of course). Sometimes, especially with young females or females with insufficient calcium in their diets, an egg may become stuck in the bird's pelvic area. The egg-bound hen may exhibit signs of swelling around the abdomen, fluffed-up feathers, straining, sitting low on the perch and/or weakness.

If your pet bird becomes egg bound, handle her very gently; crushing the egg in her will cause death. It is important to place the bird in a humid environment as soon as possible to promote passing of the egg. This can be accomplished by putting the bird in a bathroom where the shower has been running to produce steam, or by placing a steam vaporizer near the bird's cage. If the egg has not passed within a few hours, take the bird to the avian vet immediately.