European starlings look nothing like penguins. But these chunky black birds are natural-born swimmers, a new study reveals. The finding may hold key clues to how their bigger black-and-white cousins—and scores of other water-loving birds—gained their ability to swim.
The work is “exciting” and gives scientists a “new direction” for studying the evolution of swimming in birds, says Frank Fish, an integrative biologist at West Chester University who was not involved with the research.
The transition from air to water was a gradual one for most aquatic birds, scientists believe. Land-based flyers began by splashing—and eventually swimming—on the surfaces of lakes, ponds, and other bodies of water. (For example, when necessary, eagles, ospreys, and songbirds can swim along the surface.) Many adaptations later, some birds began to not only stick their heads under water, but also plunge the rest of their body downward.
This shift from swimming to diving evolved at least 10 times in birds over the past 100 million years, giving the world puffins and penguins. But none of these transitions was thought to be easy: Water is 800 times denser than air and 60 times as viscous.
To explore how birds may have first taken to water, University of Montana, Missoula, graduate student Anthony Lapsansky and colleagues collected nine starlings (Sturnus vulgaris), choosing them to study because they are common invasive birds in the United States. There was no evidence that this species could swim.
First, the researchers placed the starlings on the water of a 3.7-meter-long, 3500-liter tank. “I thought they would flap along the surface of the water to get out,” he recalls. “But they kept flying right out.”
So the researchers decided to dunk the birds in the water, simulating a fall from the sky. To their surprise, the starlings swam just as well as diving birds (see video above), Lapsansky reports this week at the virtual annual meeting of the Society for Integrative and Comparative Biology. They gracefully moved their wings up and down as they sped to the surface.
The starling’s stroke is remarkably similar to that of puffins, murres, and other avian divers. Until now, most researchers have assumed the puffin’s slow stroke rate and flexed wings (which generate lift like helicopter blades to propel the bird through the water) were unique adaptations to life underwater. But these “unadapted” starlings did likewise, suggesting the behaviors are either innate or result from how the density of water limits how they can move, Lapsansky says.
Fish agrees. That starlings immediately flex their wings suggests the birds need to do this to overcome water’s increased density and to avoid overstressing their shoulder joint, he explains. They already know how to flex their wings to get around obstacles. Also, like puffins, the starlings used their feet to change direction, so they share another trait with diving birds.
Lapsansky and his colleagues have also found another nonswimming species, the house sparrow (Passer domesticus), swims just as well as starlings. The bottom line, he says, is that it may not have taken nearly as long as scientists thought for the transition from air to water; it’s likely that many birds winged it.
Source: Science Mag