You may also notice small yellow and black striped bees (like the one in the photo at the bottom) buzzing around the holes. These are Nomada species (cuckoo bees) who nip in and lay their own eggs in the nests of the solitary mining bee. When the nomada eggs hatch into larvae, they eat the pollen that has been carefully provided for the mining bee larvae. Very cheeky!
N.B. some of these nests belong to solitary wasps. Neither the solitary wasps, nor the solitary bees are likely to sting you.
For more information about ground nesting solitary bees and the importance of short grass and bare soil for habitat, do please watch this little video of an interview I did with Stuart Roberts, chairman of BWARS (Bees , Wasps & Ants Recording Society)
Importance of short grass for ground nesting solitary bees