Solitary bees prefer crocus over some of our other favourite bulbs. They will often spend the night folded into the petals. When the petals open they have a pollen breakfast before heading out for the day.
Muscari is another useful bulb. An early bloomer its downward facing flowers are less likely to lose their pollen in the rain. Another benefit is the flower to space ratio-each plant takes up very little room but will accommodate several bees to each stalk.
Although heather is a much bigger plant than muscari it will host a large crowd of bees-think of it as the banqueting hall of the bee world. Red flowering currant is another space grabber but is a pretty plant to have in the spring garden and bees, butterflies and hummingbirds love it.