In some instances, you will find that large breed chickens eat the straw or wood shaving bedding provided. Some scratching and pecking through fresh bedding is normal, and birds will often pick out little pieces of edible seeds if they find them in straw, but you must watch for excessive eating of bedding. This tends to be more common with younger large breed chickens, and they often grow out of the behavior, but it can lead to serious crop issues, so you must make modifications until the behavior subsides.
If they are eating straw, you can try wood shavings to see if they continue to eat their bedding, if they do, you should put them on sheets or non-woven blankets for at least a few weeks. When you try them back on straw or wood shavings, be sure to watch them closely for signs that they are still eating their bedding. Consider adding new enrichment to their habitat to distract them.
Clean up with sheets and blankets is more labor intensive than using straw or wood chips- be prepared to do a lot of laundry and change the bedding multiple times during the day because it will get dirty very fast. However, the extra work is worth preventing serious crop issues that may require surgery and significant post-operative care if they are left to obsessively eat their bedding.