The lovingly-crafted wooden creations have been appearing in Newport, including one in the shape of a person with the epithet 'Bird Man'.
That one was originally put up on an electric pole but was moved to a nearby tree after safety concerns were raised.
The identity of the kind-hearted artist remains a mystery but people have been eagerly looking out for new bird boxes and sharing photos on social media.
Newport's mayor Peter Scott said: "The bird boxes started to spring up about two months ago with the one behind The Larches that looks like a post box.
"Since then at least another half dozen have popped up. No one ever sees who is doing it.
"It’s a mystery but it’s also a bit of fun and has made many people smile. Exactly what is needed in these serious times.
"He/she was christened Bird Box Banksy on Facebook. Where will they pop up next?"