Oh, how observant you are, Jennifer. The purpose of the green grate was to keep it out of a puddle that had formed on our flat roof. We've since had it fixed but at the time it was a concern about the wood base standing in water.
No real bee reason for it, other than wanting to keep all bee wood dry.
That photo was taken just as they were beginning to swarm. I LOVE being in the middle of a swarm. Makes my soul sing.