We have a flock of three so far (more in the brooder) who get along great, and have been in the same roomy coop with plenty of roosts since they were chicks.
They're about 17 weeks now. Out of the blue, the second hen in the pecking order, (Joules a barred rock), was not in the coop with the other two when we went out to close their door around 8 pm. The others were very quiet. We looked all over with flashlights and called for her but no sounds. The next morning we found Joules in the big fir tree near the coop. They all have roosted in this tree before during the day but it's not their favorite place. Now, every evening, I watch the flock do their routine of ambling and snacking on the way to the coop, walking up the ramp, and then all three going in. About 30 seconds later, I see Joules walk back out. Joules walks back and forth, and calls to the head hen, Roxie (a barred rock also). Joules eyes the fir tree and either hops the fence or walks along the fence out the gate to roost up in the tree. She's done this every evening for four days now, despite being taken out and placed back in the coop by us.
Nothing we know has changed in the coop or backyard. I'm just wondering why she'd rather roost in a tree by herself, where it's not as safe, rather than with her flock in a clean dry coop. Also wondering if we're going to have to pluck her out of the tree every single evening.
Another possibility is that Joules gets nervous in the dark coop in the evening. It sounds silly, but sometimes young birds who have been raised beneath a heat lamp can actually be afraid of the dark: a heat lamp that's on 24-7 produces light as well as heat, and so the darkness might just be alien to her. Joules might simply find it more comfortable outside where it's a little brighter for longer, or where she has moonlight (or street lights, depending on where you live). So, you might try putting a little tap light inside the coop, and turn it on before bedtime. When they're settled on the roost, you can go turn it out. A small solar light will slowly dim and go out on its own. (Generally, you don't want to abruptly turn the lights out on your birds in the evening, because it can be difficult for them to safely roost when the lights go out suddenly. Do make sure they're safely roosting before turning the light out, or make sure to use a light that dims slowly.)
Lastly, it's possible Joules is being accosted by something in the coop that you're not seeing. For instance, a few years ago, I was having a similar issue. While my girls weren't roosting in trees, several decided to roost on a stone wall beside the coop. I couldn't figure it out at first. What I eventually discovered was that they were suffering from mites; some mites emerge and feed chiefly in the evening, just when you see your chickens coming home to roost. Joules's rejection of the coop may be reaction to an "attack" that isn't immediately obvious to you. Read more about mites---and how to treat your girls---on our website.
Please write back and let us know how these suggestions work out for you! I hope the advice helps. Do my readers have any other suggestions?