Why do Geese honk repeatedly while migrating? And on long flights?

Basically, honking allows geese to maintain communication during long flights. Most importantly, it helps them to avoid midair collisions.
Also, the call and response of birds is the main reason for flying in "V-formation." The theories about the etiology of (cause) V-formations range from greater aerodynamics to superior defense against predators to better vision or auditory communication.
Another Ornithological (bird) expert told us that honking doesn't sap geese of vital energy: "They honk while exhaling, which they obviously have to do anyway."
http://birdcommunications.org/