How do budgies behave when they are ready to breed?

Budgies which are getting close to laying an egg become more lively, excitable and playful. If they have a nesting box, they will spend a lot more time in the box, preparing it for nesting, and possibly being very protective of that area. If there is a male, he may start feeding the female.

Budgies look the same when they are laying eggs as when they aren't. In other Words, birds don't "hold" their eggs inside their bodies and show a big belly to indicate that eggs are on the way. The egg shell is the last thing that develops right before the egg is laid, therefore, a hen doesn't "show" in the sense that human women "show" when they are pregnant.

Swelling in the abdominal area could also mean the parrot is egg-bound, though this tends to present more as swelling near the bird's tail, near the cloacae. She will be distressed and show signs of constipation, be reluctant to move and "waddle" like a penguin. If your bird shows signs of being egg-bound, she must be taken to the vet as soon as possible, as this can be a life-threatening condition.