Over the past few breeding seasons I have tried all manor of ways to increase the humidity in the bird nesting boxes.
I had completely changed the design of the boxes making them larger, I also removed the concaves and went to a straight saw dust base for the eggs to be laid on and incubated. I also added an open water drinker baths so the birds could have access to bathing water if they needed it. Thinking they would know what was needed to get the correct incubating humidity. ( maybe they have lost a bit of that through domestication) I have also used a mist spray to add moisture to the nest boxes when the eggs are due to hatch.
It was hard to see any improvement in the hatchability.
THE NEW PLAN.
I own a brick saw so I decided to cut my nesting blocks out of clay pavers, I knew that Clay Bricks were very absorbent so I soaked the newly cut brick blocks for a couple of days is water, then place them in nests with eggs that are being incubated. I definitely have seen an improvement is hatch ability. I am still playing with nest boxes and have a number of my older boxes back on the cages and they are working a treat. The blocks only need to be swapped over every week as they slowly let moisture vapour escape into the nest box because the bricksy hold a large amount of deep internal moisture that slowly releases
Check out the accompanying photos, most importantly the nest box itself does not need any modification. The Bricks or pavers can be cut with an angle grinder with a masonry cutting wheel.
(C) Rob Manvell. please ask for permission before publication