If the girls are not in there yet, I would consider going the chemical approach and removing all the bodies and nest parts before moving the chickens in. I've only ever dealt with paper wasps and yellowjackets, who can be nasty enough, but I heard that hornets can be quite dangerous.
If they're like wasps, the go back to the nest at night. I'd hit them in the coolest part of the day when they're most sluggish--very, very early morning. The chemical wasp/hornet sprays have a quick knock-down and kill, but they could still sting multiple times while dying so you don't want to be anywhere that they may fall. They can be very protective of the nest, and unlike bees, who only sting once, wasps and hornets can get you many times.
I was stung three times in 6 weeks one summer by paper wasps, and now I kill all colonies of wasps and yellowjackets near my house or areas frequented by my family. All bees, honeybees, bumblebees, mason bees, and mud dauber wasps are very welcome, but the nasties are DEAD. I didn't enjoy my steroid and antihistimine treatments, nor the intense heat, swelling, and itching that came from the multiple stings.