…when they are flying overhead. My six year old asked and I thought is was a good question. We had a huge flock fly over this morning. It seems like a great waste of energy considering these are big birds and probably have a tight energy budget. The only thing I could think of is that they are identifying themselves as part of a family or group within the larger flock.
Because the lane is blocked and they lack a decent passing gear. 
OK, sorry. I had read somewhere that the honking is done by the trailing geese and it encourages the lead goose (the “v” lead changes often) to stay in front as long as possible. There’s a big advantage to staying in the wake of the leader and apparently honking takes less energy than working alone or leading.
I’m trying to remember where I read this so please take it as a fuzzy endorsement of a theaory.
Hey, I’m going to put the breaks on my answer. My google skimming turns up nothing more than an often repeated anecdote about what we can learn from geese. Hardly definitive proof.
So, like you solkoe, Id like to here from a wildlife expert on this one.
I’ve never heard of this. I suspect this is just one of those “inspirational” urban-legend type stories.
Geese probably honk so much in order to inform one another of their positions and to maintain flock cohesion. It is possible that family members may be able to recognize one another’s calls and thus stay together.
This current thread has more information about goose flocking behavior, in particular on the energy savings involved in leading versus following.
This article I cited in that thread has information about the possible benefits of flocking with related individuals:
We have a lot of wintering snow geese. The members of the flock are always muttering to each other when on the ground. I think they are plotting against us. And when the are frightened and take off there’s a tremendous din.
If you want a good watchdog, don’t get a dog. Get some geese or ducks.
Colibri, I bailed out of my response before you could correct it. I think I should at least get style points for that.
BTW, I will continue honking at you from your rear flank for you to fight ignorance.
They honk, lacking an upper beam flash signal, to tell the slower moving goose ahead to yield the lane. In time, the angry goose will likely start a Pit thread about stupid slow-moving geese who don’t move, and other geese will defend their right to slow flying right where they are, and the thread will descend into a flame war where someone’s goose gets cooked. 
maybe they are honking to say “hey, I’m behind you, don’t fart.”
are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet? are we there yet?