White Zebra FInches

Question
As you know by now, I have two white Zebra Finches.  Up "until" now, both were thought to be girls.  Because neither one has utter a song of any kind, I tend to think this is true. But, recently, while one sits on her eggs, the other is busy constructing a nest around her.  And when she leaves the nest, the other one is busy forming a nest from things I have given them in the cage to do so.  I give the bird pieces cotton fiber nesting material I purchased at the feed store for finches, and it uses it all.  Will a female Zebra Finch construct a nest?  Or do I have a male and a female?

Answer
It's hard to tell just by the description but males usually have orange cheek patches whereas females don't. All the males I have ever had have always had cheek patches but if you still want to be sure I would try this method:
Take the bird and blow on the rear (I believe it's called a vent) and look at how the feathers grow around it.
If you have a bare belly with no feathers around the vent than you have a female.
If you have a feathered belly with long "feeler feathers" around the vent then you have a male.

This isn't a guaranteed method. The only way to be 100% sure is to get the birds DNA tested