Moving your chickens to fresh ground gives your birds new grass every few days when they can’t free range. Keeping them safe and the tractor ‘critter proof’ is obviously very important, as is being able to access the house part easily to collect eggs and for cleaning purposes.
An interesting example is shown in the video below. Built primarily with 2x4s, hardware wire cloth, and some left over shingles it also has wheels and a handle for moving it to new ground and also a hole for ventilation, and shade considerations.
The girls should be happy