From Cornell’s BirdCast Project:
Migration Amounts (pre-2018)
For ease of describing bird densities and migration traffic from weather surveillance radar (WSR-88D in particular), BirdCast has chosen to divide movements among four different classes. We provide a qualitative description, a relative measure in decibels of Z (dBZ) of the energy returned to a radar, and a bird density based on previous work by Gauthreaux et al. (1999). Additionally, see this article published previously.
Little to no migration — Minimal migration: < 5 dBZ — fewer than 59 birds per cubic kilometer
Light — Light migration: 5-10 dBZ — approximating 59-71 birds per cubic kilometer
Moderate — Moderate migration: 10-20 dBZ — approximating 71-227 birds per cubic kilometer
Heavy — Heavy migration: 20-30 dBZ — approximating 227-1788 birds per cubic kilometer
Very Heavy — Extreme: >30 dBZ — more than 1788 birds per cubic kilometer (this intensity of movement occurs rarely, perhaps at certain individual radar stations a handful of times each migration season)