Hi Lisa and welcome to GoY, I would have thought you ran the risk of having vermin getting into the compost to eat the hen food. Mind you I am surprised there is any food left our girls used to eat everything we gave them.

Certainly do not think left over hen food is a good idea, you definitely would get vermin in the heap.

It might get smelly, too, since most chicken feed is high in protein.

Agreed Tugb.

If you have un-eaten food in the chicken run then might I suggest that you are overfeeding, Lisa? Even in full-lay chickens should be able to eat up all the food that you put down and then have a good scratch about in the run. The un-eaten food in the run could be an attraction for rats; not something that you want.