Who is eating the Orchard Mason Bees?

Here are the mud-plugs carefully applied by orchard mason bees last summer. I noticed today that about 1/4 of them are poked out. Is it another insect? Birds?

Orchard Mason Bees are non-honey making bees that do not have the highly organized social structure of honey bees. They are efficient pollenizers of fruits (which is why I started playing with them). Honey bees are declining due to mites, and there is some thought that orchard mason bees will be needed in greater numbers to pollenate fruit trees.

There may be local bees anyway - they love the ornamental cherries. However, I bought a kit from Raintree Nursery (mail order), set it up, and even carefully left a bowl of mud near the 'bee house'. It was fun watching the enter and leave the little tunnels in the house. If only 1/5 of the tunnels remain intact, and those bees survive, it will be an increase over the original population.

Still, who is eating them?