This is what you need.
I made my own.
The trouble with baby bluebirds disappearing is crows/black birds with narrows heads and long beaks.
It took me 2 hatches of bluebirds to figure this out then made my own guard.
With a guard those larger birds can't get close enough in inside to destroy the eggs/baby birds.
And don't believe the crap above about not feeding the bluebirds.
Enjoy watching the birds with meal worms close to your house and enjoy their song.
https://www.amazon.com/s?k=bluebird+..._ts-doa-p_1_21