Every evening here we'll get a flock in trees, not always the same tree every night, but they'll find one to gather in. Sometimes a couple go to one and a couple to another and then they all end up in one or two trees next to each other. Apparently some crows will migrate, others will have winter feeding grounds. This video I found looks like the way yours were flying once you get into it.
There is a resident crow flock that roost in the trees on the west side of the parking lot-great view of our dumpster-but it's probably 20-30 individuals. It's the absolute silence that has me puzzled. Crows & ravens are a noisy bunch even in flight.

















