I've noticed the same thing. At almost exactly 11:30 a.m. the bees leave the buckwheat. You'd be surprised how close it is to that time each day when they go elsewhere.I give you the example of Buckwheat, which when conditions are right, produces prolifically in the morning but shuts down by early noon. Bees will work buckwheat , then shift to a less attractive nectar/ pollen source only because the buckwheat shutdown. Oh and the girls will be a bit miffed when the buckwheat does first shut down.