In the early spring they often go hungry, therefore planting a few early nectar rich plants is important for their survival.
Hellebores are are brilliant. Ivy is notably one of the best forms of nectar. Catkins are excellent so that would indicate hazel.willow and garrya elliptica. Without double these are important plants to include in your garden, not only are they vital for bees they also provide great winter and early spring interest.
More on helping bees >>http://www.thenurseries.com/help-bumble-bees-p-2326.html
Save the Bees in Early Spring >>