Why Do Loons Belly Flop When They Land?
Common Loon; photo by J-Photos/Unsplash.
We’ve probably all seen––or seen video of––ducks or geese coming in for a landing. Like water planes, they touch down on the surface of a lake or pond with their stubby legs held stiffly at an angle to help them brake. The broad surface area of their webbed feet creates friction on the water until they lose enough speed to settle their bellies onto the surface. Then, they shift from water plane to boat, their landing-gear feet now ready for paddling.
A loon, however, does not land like a duck. At all.
Rather than water planes, loons touch down on lakes and oceans like projectiles. Their toes do touch the water to create friction, but their feet are held backward from their bodies and bear none of their weight. Instead, their bellies take the brunt of the landing. Once they’ve slowed enough to settle in the water, they sit low and are ready to dive and swim under the surface, like feathered submarines.
So, why do loons land on their bellies? Unlike so many other waterbirds?
The answer lies in the bizarre structure of their legs...because loons use their legs differently than other waterbirds once they’re in the water.
A 2017 study in the journal Anatomy examined the trade-offs that birds like loons face when their bodies are highly specialized for diving––versus waterbirds that spend most of their time swimming on the surface.
Compared to those of ducks, geese, and swans, loon legs are positioned extremely far back on their bodies, much closer to their tails than to their fronts.
But that’s not all.
Loon legs also have far less flexible hips and knees than other waterbirds. In fact, those joints are technically inside their bellies (see Fig. 2 of Clifton et al. 2017, below). The hip and knee are tucked up next to the ribs, rather than extending outside the body wall. This combination of features means that loons can barely stand, let alone walk or put weight on those legs for landing.
Figure 2 from Clifton et al. 2017 in the journal Anatomy comparing the leg morphology of waterbirds. A surface-swimming duck has a very flexible leg, located pretty much at the bird's center of gravity. A loon does not have a lot of flexibility in its hip or knee, and its foot is a lot closer to its back end than its front end, making a legs-out water landing impractical.
Having legs positioned mostly inside their bodies, and really close to their tails, may make loons highly unusual landers, but the upside is that it also makes them exceptional divers. And for them, the trade-off is obviously worth it.