I don't know about magnolias. Why can't you use them in compost?
That said, leaves in general are an excellent, free resource for deep litter in the coop and/or run. The chickens will poop in it and scratch/mix it up for you. If exposed to water (generally run), then it will break down into an excellent compost for use in your garden. If kept dry (generally coop), then it will still be well broken down into smaller particles, but not fully composted by worms/bacteria.
Another benefit is that the high-carbon leaves do an excellent job of holding down odor from the high-nitrogen poo. With deep litter, I only change my litter twice/year and it then goes into my garden and/or under some bushes & trees as excellent organic compost. This last year, I produced enough extra to sell to my neighbor for her raised beds and she had her best garden year.