Actually they were chocking there chicken with them.
A few years back I had a cedar tree in a fence row I needed to trim. In doing so I found an empty bird's nest with a blue plastic grocery sack neatly woven into the "floor" of the nest. I took the nest down and put it away in the shed. A couple of years later the old cedar was looking really rough and I decided to cut it down. While cutting the boughs to length for disposal I found 2 more bird's nests. They were like the one I had found previously, with grocery sacks neatly woven into the floor of the nest, and one actually had strips of bags cut and woven into the side. I was amazed, but then realized the birds found shopping at the neighborhood Walmart convenient, too.