Mailbox nest?

Over the past few weeks I have noticed grass and small branch sticks inside the box I built around my mailbox.

I had built the box to protect it from the snow that the snowplow clears from the road.  At 35+ mph the snow goes flying.  The county doesn't pay for the damage the snow from their snowplows can cause to mailboxes.  Over the years my mailbox got bent from the snow until I had to replace it.  Then I put boards up on the sides.  I then had to make the "protection" a box to protect the "protection".  And then a roof on the box as the falling snow would then sit around the mailbox.

A few weeks ago I noticed grass and sticks up near the front of the box.  I would pull them out.  And more sticks and grass would show up days later.  The area is small.  I can barely get my arm in the area to remove the sticks and grass.  Today I had to use a crowbar so I could reach all the way to the back of the box.  Some of the grass is quite long.  If there is grass behind the mailbox I can't reach it.

The post is a metal pipe.  So I don't think a mouse is trying to make a nest.  So possibly a small bird is doing this.  Why?  Isn't there a better place to make a nest?