Female budgie going in and out of nest box?

I have 4 budgies that are paired up in a big cage with 2 nest boxes. They preen each other, clean each other, and the male always gives food to female. All budgies are 12 months old. Recently I Have noticed the females going in and out of the nest box ALLOT!! Sometimes they stay there for a few mins while the male is guarding them outside. Both females have their own nest boxes. What does this mean? Does it mean they want to lay eggs? When do they breed? I have had the nest box for a week now.