Red Mason bee?

Red Mason bee?

Observation - Red Mason bee? - UK and Ireland. Description: I saw about 20 of these coming back & forth to a robinia tree in my garden. Some were crawling into holes in the tree, where they would stay for a short while, then fly away again. My initial thought is that they are red mason bees, but they also look

I saw about 20 of these coming back & forth to a robinia tree in my garden. Some were crawling into holes in the tree, where they would stay for a short while, then fly away again.

My initial thought is that they are red mason bees, but they also look a bit like tawny mining bees. However, I thought those are solitary bees. Anyone got any ideas?