How do you know when a budgie is about to lay?

Answer 1

Budgies are birds, not mammals. They do not have the same kind of reproductive cycle as mammals. Budgies do not menstruate or have periods. It's not like having an un-spayed female dog or cat that goes into heat occasionally.
Wild budgies are what are called opportunistic breeders. They will seek a mate whenever there are enough resources to support offspring. In captivity, a properly cared for budgie will always feel that she has enough resources to have babies. This is why it is important not to house budgies of different sexes together unless you are prepared for the possibility of chicks.
Even without being housed with males, a female budgie will sometimes produce an (unfertilized) egg. Budgies don't have the same genetics or diet as domestic chickens, and excessive egg laying can cause serious health problems including calcium deficiency and egg-binding. You should consult an avian veterinarian for more information about budgie reproduction.

Answer 2

Budgies which are getting close to laying an egg become more lively, excitable and playful, not listless and quiet. 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. If no nestbox is available, the female will make do with what she has, perhaps preparing a corner of her cage and spending more time down there.

Budgies ready to lay do not get fat. Any parrot with a swollen abdomen is not a sign of of a bird about to lay an egg. It is either eating too much of a high-fat diet (common among birds with a seed-only diet) or it has a tumour.

Swelling in the abdonimal 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.