The first bee you see this spring will probably be a bumble bee. Female bumble bees emerge from their underground nests earlier in the spring than other bees. Their large bodies and ability to regulate their body temperature by shivering and basking in the sun keeps them active on cool days in spring and late into fall. Look for them on early blooming plants like willows and late blooming flowers like goldenrod. The bumble bee will collect pollen in her pollen basket on her hind leg and carry it back to her nest to feed her babies.
The flower fly, Volucella, has learned to mimic the bumble bee so that it can enter the bumble bees nest and lay eggs. The fly larva act as nets cleaners, feeding on dead bees and other detritus. Thus, the bumble bee colony benefits from its guests / house cleaners and the fly larva have the protection of the bumble bee colony.
To find a bumble bee nest keep your eyes on the ground. They are generally located in bare patches of soil that are well drained and on a slight slope. The entrance hole is as small as the bee and may sometimes have a pile of excavated soil next to it like an ant nest. Watch for bees flying low to the ground in a zig zag pattern and watch for them to land. Keep a few patches of undisturbed bare soil in your yard to invite bumble bees to nest.