Originally Posted by
etudiant
Surprised that people are unaware that birds migrate mostly at night. They use the day to feed. The nexrad radar in the US gives a ringside seat. One can watch the masses of birds take off at dusk and gradually settle in near dawn.
UW-Madison Department of Atmospheric and Oceanic Sciences
Depends what sort of bird & where. Some routes they use thermals to help climb so obviously start during the morning.