These tiny little bees, 6-7mm long, nest in in burrows in soil, woody, dead stems and also in wall crevices.
Unusually, these bees collect nectar and pollen in their crops, (I didn't know bees had crops), and regurgitate it when they get back to their nest.
This one was gathering food from a Dandelion Taraxacum officinale, a native wildflower.