I watched that video and subscribed to the channel - I really like her. It strikes me that Australia does much better than at least the US but maybe also UK on sensible chicken vet advice - and even the assumption that you can find an avian vet.It's been a week today since the vet called me to say Caramel didn't survive. When she was in full health she sang a gorgeous baritone egg song every day. This is a short clip of her song a year ago, before we had the rooster.
In her last month of life she was spending an hour every morning in the nest box singing with a completely different small, high pitched voice, but never laying.
Someone with more experience would maybe have thought that her trouble breathing and her behaviour in the nest box were linked and adding to probable reproductive problem. It is very well explained in this video @MaryJanet posted once.