Duck Math - addition and subtraction

Have you noticed the number of ducks keeps changing day to day? Momma goes, Momma returns. One day we were down to one guy! But since then I've seen all 3. They cross the street on "foot" or by flying. One was recently dawdling as he walked across, but when brother just decided to fly across, the dawdler leapt up and followed. About a week ago I was returning around 5pm from the direction of the library and suddenly noticed a huge bird flying towards me on Bedford Road. It was a flying Muscovy, heavy on the white. When I parked at home I saw that Momma Duck was not there, so it could have been her navigating the street like a flying car.