Hi Suzanne,
Birds do not actually carry eggs inside them, because all of the incubation takes place outside of the body. It takes about 48 hours for each egg to form, move through the reproductive track and be laid. If the hen has mated recently, the egg is fertilized during this process. But of course she can lay an egg whether it is fertile or not. The egg must be incubated around the clock for at least 48 hours before any development begins. If it isn’t incubated, it will be just like any unfertilized egg. Budgies lay 4-6 eggs, every other day. Mating does not have to result in eggs. Some pairs will mate and the hen never lays eggs. If she starts spending a lot of time in the nest box, and they are mating frequently, these are signs that she may be ready to nest and lay eggs. There are not any physical changes you will see before she lays an egg as long as everything is going well. If you see her on the cage floor, fluffed up with her tail pumping up and down, she is probably egg bound and would need to be taken to an Avian Vet immediately. This is why feeding her a balanced diet – not a seed mix – is very important. Also offer dark leafy greens, chopped veggies and some fruit. If she starts using the nest box a lot, you can start feeding her an egg food daily. Either a dry commercial egg food or cook an egg with the shell washed, crushed and cooked with the egg. This provides protein and calcium that she needs to form healthy eggs. You can also give her a cuttlebone.
Thanks for the updates!
Brenda