The sky is full of them, night and day. At night one can hear their honking overhead presumably so they can stay together flying in the dark.
But they honk as they fly in daylight too, often in huge aerial V-shaped rafts of scores of birds. I can't be sure, but it sounds as if the leading birds do the honking.
I wonder what the purpose of of the sound is. We know from the recent studies of young captive birds guided in their first migration by humans (sometimes in ultra-light aircraft) View this photo ) that the young birds learn their life-long migratory pathways by imprinting on landmarks along the way. I wonder if they ever get lost flying at night?