No doubt, this was a trained duck, you could see the rings on its feet. A duck would fear a stranger approaching him, and would surely run away.
A goose is different, it might attack, as they are more related to swans, which are aggressive and territorial.
Legend has it that the Roman geese have alerted the guards when the Gauls were at the gates, and so saved the temple of Juno. The Gauls have already managed to sack the rest of the city of Rome, but have lost the element of surprise by the loud honking protest of the temple's resident flock of geese. The temple guard was immediately alerted, and repelled the invaders.
I read somewhere that the American military have used geese as guards, around some Air Defense installations in West Germany, during the cold war. No, they did not equip their geese with firearms

, but relied on their honking to raise the alarm

.