If the nest box is 18" off the ground...

[CityGirlintheCountry]

and that is where my broody has chosen to lay and, well, brood, what happens when the chicks get here? Do I need to make a new nest on the floor? Will my mama hen stay there or is she going to want to be back up in the chosen nest? She seems pretty darn attached to it right now. I'm worried about the babies falling out and not being able to get back in.

 

[2468Chickensrgr8]

Thats a tough one...I'll tell you my Experience.....my little one decided she was going to hatch out some eggs...in her favourite nest boxes BUT all the other girls decided thats were they were going to lay their eggs also....so after asking the same Questions I decided to get a small dog crate(then later after they hatched a larger one) and cut the bottom out so I could place it over her...and I moved her to the corner on the floor with food and water...when all the girls were outside I would open the dog cage door so she would have the opportunity to walk around etc...sometimes I would have to hand feed her...When the eggs hatched I moved them to a larger dog cage and I layered alot of newspaper on the bottom so I could clean it easily and I wired some wood around the bottom so the big girls couldnt peck them but could see them..and the little ones couldnt get out ....then when they were bigger I let them go with mom an hour or more a day and increased the time...worked for me...hope I helped...
My broody didnt mind me picking her up....its funny because she was the most skittish chicken I have and I was kind of worried she would stop.

 

[Red]

My hen went broody in her nest box in the coop, it was about the same height as what you are describing. We were also worried about the chicks falling from the drop and either injuring themselves or getting stuck down there; then there was the fact that the nest was shared by two other older hens too.

We built a small 'broody' coop for our girl (an old rabbit hutch with a small run with food & a dust bathing area) and moved her into that at night. She loved her new nest and didn't mind being moved at all. She seemed relieved to have her own place away from the other two chickens, and it was much safer and more contained for the chicks as well.

Good luck in whatever you decide to do