This is normal o breed budgies and she is preparing for eggs they will pluck there vent feathers as part of the nest to help keep the babies warm Happy Breeding!

I had setup my breeding pair about 2 weeks ago and everything was looking fine they started mating in 3 day's and the female spends most of the time in the nest box. but recently i have noticed that the female is plucking her vent feathers, she isnt showing any signs of egg binding and is spending 2/3 of the day in the nestbox her poop is huge just like usual and isnt watery or abnormal , she doesnt sit on the floor