I was monitoring my garden "bee-hotel"; paying particular interest to the few Osmia leaiana nests that were completed during 2013. A hole was being gnawed in the chewed-leaf plug sealing one of the nests, so I stayed around to observe. During a one hour period, a male Osmia leaiana appeared, then a female, then a parasitic Sapyga quinquepunctata wasp. The wasp had to retreat into the hole as a female Osmia bicornis entered and started removing nest debris (see composite photo). The next time I looked, I could see that the bicornis female was nest building. I speculated that there may well be further leaiana brood cells remaining deeper in the hole and wondered what would happen to them.
Today, I noticed a hole appearing in another one of the recently-completed bicornis nest holes. I was intrigued, because all the bicornis from last year had emerged weeks ago. I waited and was interested to see what looked like a male Osmia leaiana emerging (second photo). This was definitely a bicornis nest because it was sealed with mud. It would appear that Osmia species multi-occupancy can occur in these holes. I think that after some outer-most leaiana (and cleptoparasites!) have emerged, holes are "high-jacked" by bicornis females who utilise the vacated portion of the hole. The male leaiana that I saw emerging from the second hole, would (presumably), have had to break through several bicornis brood cells to get to the surface. Has anyone else witnessed this?