Many of these small bees were frequenting the blossoms of the wild strawberries (Fragaria virginiana) which we keep as a groundcover in our front yard. The bees are never still for more than a few seconds, so they are hard to photograph. We think they are the same bees we noticed nesting in our switchgrass stems, about 40ft away. The flowers of wild strawberry range from 6 to 25mm wide, and these bees were 1/3 to 1/4 the width of the flowers. The two photos are of the same individual, different angles.