I maybe wrong but l think it was in the late 90’s when buff feathered birds were first appearing and breeders were trying to introduce this type feather into this countries budgerigars. The birds of today now seem to have much more feather and it has been reported it does not come without problems.
Due to the density of the down feathers budgerigars of today need a little help in get more full eggs, one way is to trim the vent feather prior to pairing up which will allow the cocks sperm to reach the hen.
Something else l was told was to lock the hen out the nestbox for at least one hour a day to allow the cock to tread the hen it is something l failed to do last year and a couple of pairs only filled the first two eggs and the maiden hens seem to refuse to leave the nestbox so this year l will be locking my hens out for one hour a day when l come home from work.
Livestock breeding is not exact science but simple tips shared by experience breeder really do help myself and others but we must always remember what works for one breeder may not work for you.