Dogs are so weird lol. I'm glad she's not harming the hen, but I do think you want to stop this behavior before it becomes an obsession.
When I got my chickens, I would bring my dog out back on a leash and walk far away from the chickens, just walking and not drawing attention to the chickens. He knows the "leave it" command (very useful and I recommend teaching it if she doesn't know it already). Every time he would pay too much attention I would give him a stern "leave it", and reward him if he did leave it alone, and then go back to distracting him. Any time he WASN'T obsessing over the chickens or paying attention to them, I just would act like I was the most fun dog mom ever and we would play, run, have fun, and snuggle. But if he paid attention to the chickens, he'd get a "leave it" and we'd go back to distraction time. Pretty much letting him know that we can do other things in the yard while chickens are there. Then, once he's good at that distance, do the same thing closer and closer until he no longer cares about the chickens. Basically, you're trying to desensitize the dog and get them to feel that the chickens are no big deal and are pretty much just part of the scenery. It worked for me. Once I could trust him on-leash, I let him go off-leash but kept his e-collar on just in case. I only had to use it a couple times and now he just ignores the chickens.