It has long been known that large flocks of birds could be seen on radar, but with the advent of doppler weather radar deployed nationwide, this capability has come into full maturity. Overt the past 20 years, the BirdCast program has matured and uses this weather surveillance radar to gather information on the numbers, flight directions, speeds and altitudes of birds in flight every day. This has provided great insight into bird migration patterns.
Today we can go online at https://birdcast.info/ at get real-time predictions of bird migrations: when they migrate, where they migrate, and how far they will be flying.
Dokter, A. M. 2024. BirdCast, live migration map; Cornell Lab of Ornithology. https://birdcast.info/migration-tools/live-migration-maps/.