Just goes to show that when varied pollen sources call, the bees listen. Eric Mussen was all up on that theory ages ago.
Then read Randy Oliver's Fat Bee articles:
Makes sense to me........
Pollen source - In my area Black Berry is one the last pollen sources of the year , and the saving grace for our bees. In years past Black Berry has gotten us out of many a sticky sitch during severe nectar/pollen dearths. It's very easily identified by the gray nondescript color.