For a human parallel, caged hens are like human prisons with no common areas/exercise yard. Hens in aviaries are living perpetually on a New York City sidewalk at the height of rush hour.
The article quotes a report:
Perhaps most troubling, “hen mortality was much higher in the aviary system,” the report said. When hens move around more freely, it is easier for them to spread germs. And hens in cage-free aviaries were also more aggressive than their cage-bound peers, pecking at one another and, in some instances, becoming cannibalistic.The sight of chickens pecking a hen to death is not a pretty one, take it from experience.