Thank you very much for the help. I'll notify a few friends who are equally as curious about these bees as I. We're primarily interested in birds, but also observant of other aspects of the natural world while we're afield. None of us had ever witnessed such a phenomena (the extensive aggregation of nests) before.
Is this behavior typical of the species in our area (coastal NY)? If not, what conditions could cause it this year? Did we overlook the species before only because it usually is more "solitary"? Thanks again for your help.

How about these 24,000 nests of Colletes cunicularius? and a blissfully happy bee researcher?
These bees are solitary in the sense that each one takes care of her own brood. They don't have colonies or hives, but they do have aggregations of individual nests. Jean Henri Fabre described some big nest aggregations in France more than a 100 years ago.
I would love to find something like that.
BTW they are all females. Males have nothing to do with nest building.

Yes, this is typical nesting behavior (lots of females, each making a separate nest). Colletes needs sandy soil, and I suspect such a requirement is becoming more difficult to meet as urbanization and sprawl pave over their preferred nesting locations.