So, out came the ladder and down came the box. Getting the roof off wasn't easy as the screws were badly rusted and when I took it off I wasn't sure what I was going to find inside; I knew it had been used last summer, but didn't now whether chicks were raised successfully.
But it turned out to have two nests inside - one a neat, little sparrow nest and another, a wasps' nest the size of a grapefruit. Quite a surprise.
I wonder if the sparrow chicks found that all that buzzing going on next door kept them awake. Or maybe it was soothing and lulled them to sleep.