How does the broody hen increase humidity to 70% at lockdown?

[Maria G]

This humidity thing is a mystery to me. I know all about the correct humidity settings for incubators, but how does this translate to the real world? Does the broody hen somehow change her humidity levels at day 18? Does it increase because she never leaves the nest, or what happens?

Just curious

 

[sourland]

As the eggs are about to hatch, most broody hens pull their own 'lockdown' and refuse to leave the nest. This may help to keep the humidity within desirable limits.