Just reporting back that I ordered a collapsible puppy pen with a zip open top and side door, and the 4 chicks we raised in it are pullets now and they are super friendly. I definitely think having a more open brooder where they're exposed to lots of new stimuli has made them way less fearful.
Trying to find info on this but coming up short. I'm curious what people's experiences are raising chickens in standard 'dark' brooders (i.e. a Rubbermaid bin where they only opening is on top) vs. more 'open' or clear walled brooders (i.e. glass or grating where chicks can see a lot more of...