Last week I lost one of my two oldest hens. Now her sister spends hours each day looking for her and calling for her.
She will fly up to a good vantage point and call and look around. I go out and talk to her, pet her, try to distract her with food and she just keeps yelling. This will go on for over 30 minutes, she will calm down and forage and within the hour, starts back up again.
When her other sister died she was upset, too. Just not anywhere close to this degree. These girls were incredibly bonded and it's so hard to see her so upset about it. I've got 19 chickens so it's not like she's alone.