By Helen Czerski
June 2, 2022
The River Thames is beautiful in early summer, and the stretch just upstream of central London is full of birds: motionless herons watching for fish, bustling moorhens poking around the submerged plants, and watchful flocks of preening swans. But as my canoe club paddles along the river, we’re always looking out for our clear favorites: the mother ducks with a fluffy cluster of ducklings tagging along behind.
When they’re lingering at the shore, it all looks slightly chaotic, with baby birds dawdling and exploring as the mother duck keeps an eye on them. But when it’s time to move on, chaos shifts into order as the ducklings line up behind the adult and the convoy moves off together quickly and in perfect formation. It’s mesmerizing to watch, and it’s not accidental. This is all about saving energy.
Swimming is hard work, because water is dense and slightly viscous. At the surface, a large part of the resistance to movement comes from the waves that are produced behind whatever is moving: its wake. This is the familiar wedge-shaped wave pattern that we see behind ships, canoes and ducks alike, and all of those waves carry energy. If you move along the water surface, those waves are always continually generated and so the swimmer needs to put in enough energy to create them. This energy cost is felt as a resistance to movement. There’s also resistance because the water touching the ship or duck is pulled along too, creating additional drag.
The mother duck is strong enough to expend the energy needed to overcome that resistance, creating the wake as she goes: a pattern of peaks and troughs that moves along with her. But once that water pattern is there, the ducklings can choose where they swim in it.
The wake has two parts. One is the outline of the wedge shape, the easiest bit to see, but inside that there are lines of waves that are perpendicular to the direction the mother duck is moving in, stretching from one side of the wedge to the other. The first duckling will stick at a very specific place in the centerline of that pattern: surfing downward on the front face of the first wave. It turns out that in this position, the duckling itself experiences almost no wave drag and actually get a bit of a push forwards as it surfs down the wave. As long as it keeps itself in exactly that position relative to its mother, swimming is easy. But what about the ducklings behind it?
If you look closely at a line of ducklings, you’ll see that they are spaced out very precisely. Each one is sitting one wavelength behind the one in front.
That means that each one is getting help by surfing down the front of the next wave in the chain. The front duckling gets the most assistance, but that help is passed down the chain so that even the duckling at the end experiences almost no drag due to making waves. That makes it far easier for the ducklings to swim, and allows them to move at the same speed as the adult. The energy-saving effect of the mother duck’s wake is an accident of physics, but the ducklings take full advantage of it.
Next time we’re out in the canoes, I won’t just be looking out for baby ducklings because they’re cute and fluffy and happily curious about the world. I’ll be keeping an eye out for a mother duck moving her brood by pulling them along in her wake, because it’s such a lovely example of hard-core physics helping even the smallest and most unsuspecting ducklings.
Dow Jones & Company, Inc.