If you click on the photos to see the full size images, the first photo shows one of the 3mm holes capped. The next shot shows one 10mm hole completely capped and one in progress being filled about a cm back from the edge of the hole. The third shot shows 3 10mm holes completely capped with a grey mud like substance. I assume these are mason bees in the large holes, which type I have no idea as I have not managed to see any activity. Indeed one day there was nothing apparent then the next I noticed these. This from wikipedia on the Mason bee lifecycle:-
The bees emerge from their cocoons in the spring, with males the first to come out. They remain near the nests waiting for the females. When the females emerge, the first thing they do is mate. The males die and the females begin provisioning their nests.
Osmia females like to nest in narrow holes or tubes, typically naturally occurring tubular cavities. Most commonly this means hollow twigs, but sometimes other similar spaces are used, including empty snail shells (they do not excavate their own burrows, unlike many bees). A female might inspect several potential nests before settling in.
Females then visit flowers to gather pollen and nectar, and it will take many trips to complete a pollen/nectar provision mass. Once a provision mass is complete, the bee backs into the hole and lays an egg on top of the mass. Then she creates a partition of "mud", which doubles as the back of the next cell. The process continues until she has filled the cavity. Female-destined eggs are laid in the back of the nest, and male eggs towards the front.
Once a bee has finished with a nest, she plugs the entrance to the tube, and then may seek out another nest location.
By the summer, the larva has consumed all of its provisions and begins spinning a cocoon around itself and enters the pupal stage, and the adult matures either in the fall or winter, hibernating inside its insulatory cocoon. Most Osmia species are found in places where the temperature drops below 0°C for long durations, like Canada, and they are well-adapted to cold winters.