Our pair of mutes have been breeding on our very small pond for 7 years now. From the time that nest building starts until the cygnets are a couple of weeks old, Samson is a madman and nobody--no human, no other bird--I do mean no one is allowed near his pond!! But let the cygnets get a couple weeks out, Samson starts moulting and at that point becomes extremely mellow. He's at that stage right now, in fact. One day this week we had 10 geese arrive and some even got in the water. I was watching out the upstairs window and really became concerned when I saw absolutely no sign of Samson and for goodness sakes, there were GEESE on his pond!! In fear, I went flying out the front door only to find him sound asleep in a corner I couldn't see from the window. Never have I seen him get that mellow before!!
I don't know whether the "mellow" spell is due to the baby getting older or to Samson's moulting, but we do see the same pattern every year. I have read that the pairs actually moult in succession so that one is always able to defend while the other is close to "featherless" for a while which also would seem like a logical answer to his mellow attitude. He usually stays that way for a couple of months, though, and that includes the time his mate, Delilah, is going through her moult as well.
The best geese guards we have in the end are a specific pair of Canada Geese themselves that return every year (one looks like it had a broken leg or problem that healed badly and thus is easy to identify). They are totally territorial about their area--which doesn't include the pond itself though as Samson won't let them on it.