What's happened to my Mason Bees this year?

There were loads of them swarming around the boxes in April / May and plenty of them were doing their stuff together on the ground. I took the pics below this morning as i noticed that hardly any of the holes have been sealed up. The middle box i've had for 5 years and the outer two about 2 years. It's as though they've simply "flown the nest". I wanted another three but Growing Success don't do them any more and i can't find them anywhere in stock on the interweb. Anyone else got similar situation with their Mason Bee boxes?

 

Oh dear. Sad to say it's not just me then. I think there's about 10 holes sealed up for next year so i suppose they'll just repopulate in time

 

The bees behind the shower have dissapeared too

 

Don't worry! Mason bees only appear in Spring, this is completely normal, they will be back next year on cue!

 

They might have gone home.

http://tardis.wikia.com/wiki/Melissa_Majoria

 

Yes i suppose so but from year one all the holes in the boxes have been jam packed - and the keyhole in the shed door, anywhere there's a hole to lay eggs in fact.

 

Maybe they've moved on.. What if the queen has died and there are no more? You might get more from a different colony next year?

Turns out that they don't have a queen... Ok that's quite strange then. I'm sure they will be back from somewhere next year