Laying while roosting

[Misty Farm]

I have six hens. Two Americana, one black australorp, three isa brown. They are two years old and each lay an egg a day.
One chicken is laying her egg in the day while roosting. Not sure which hen but not the Americanas. This is new or different behavior from last year.
I have three nest boxes, coop is 8x10x6 with an outside run. Roost is about 4 foot high and they all use it. There are two lower roosts. I find the egg later in the day beneath the high roost. Sometimes broken but not eaten. Egg appears normal.
Feed is crumbles, scratch and oyster shell. No changes. They also free range most days.
Is it possible to encourage a hen to lay in the nest box? Or discourage from laying while roosting?

 

[rosemarythyme]

Have you actually seen her (or any of the birds) roosting inside during the day? I would think it's more likely she's chosen that corner under the roost to lay. Shell thickness okay on the eggs?

IDing exactly which bird it is would be the next step, so we could see about either supplementing calcium (if egg shells are thin and causing breakage) or encouraging her to use the nest boxes.