This morning, the 60 strong flock of Canada Geese that seemed to have roosted in the park were restless. There were continuous loud grunts and honks and, after a crescendo in which more and more individuals joined in the calling, part of the flock took off in a coordinated way. The vocalising geese were making a decision, with individuals deciding to join or not a party leading the departure, maybe to quieter feeding grounds. It is unclear if the individuals felt hungry or if the several dog walkers had made then nervous (or both!).
Flocking birds have to travel together to stay in a group to search for new feeding or roosting patches, to avoid predation or to migrate. When the individuals differ in their motivations to move away (some might be hungrier than others, for example), there might be a period in which an a consensus is reached about leaving or staying involving communication of the individuals intentions. An individual that is not hungry might change its mind and join in if most of the flock appear willing to move, so as not to become isolated.
Dennis Raveling, in a paper in 1969, reported on the behaviour before flock taking off in Canada Geese. He observed a large flock in their natural range, in which 77 geese had been marked and radio tracked, including 10 families. Flock departure was preceded by a ceremony, with the neck stretched, there are quick head tossing movements with the bill pointing up and repeatedly, and the white head patch conspicuously displayed - communicating an intention to fly. Geese often spread and flap their wings and start to walk in the intended direction of flight for a few steps (this video illustrate this behavior). Ganders (adult male geese) were more successful at recruiting his family than any other family members, as a shorted time elapsed from his initiation of head-tossing until the family took flight, although all family members initiated head tossing at some point. In a couple of occasions when an excited immature took flight but the rest of the family did not follow, it flew in a circle and returned with the family shortly.
Other than the cohesive function of the head-toss ceremony for family members, group vocalisations and wing flapping serves to synchronise the whole flock. The coordination of the flock during take off included the presence of an invisible 'starting line': individuals run until they arrived at the same position at which the individual before them had just taken off (instead of taking off where they were). Raveling hypothesized that the contrast between the white tail coverts and the black tail served as a signal to optimise the position of individuals during flight, quite important in such large birds. Families tended to keep together during flight and their vocalisations then changed from grunting to a more trumpet-like honking.
Dennis G. Raveling (1969). Preflight and Flight Behavior of Canada Geese The Auk, 86, 671-681 DOI: 10.2307/4083454