By Elizabeth Pennisi
European starlings do not look like penguins. But these heavy black birds are naturally born swimmers, according to a new study. The finding could contain important clues as to how their larger black-and-white cousins and numerous other water lovers learned their ability to swim.
The work is ‘exciting’ and gives scientists a ‘new direction’ to study the evolution of swimming in birds, says Frank Fish, an integrative biologist at West Chester University who was not involved in the research.
Scientists believe the transition from air to water was gradual for most waterfowl. Landslides began by splashing on the surfaces of lakes, ponds and other bodies of water – and eventually swimming. (If necessary, for example, eagles, ospreys, and songbirds can swim across the surface.) Many adaptations later, some birds not only began to submerge their heads, but also submerged the rest of their bodies.
This shift from swimming to diving has evolved at least ten times in birds over the past 100 million years, giving the world parrots and penguins. But none of these transitions were easily considered: water is 800 times denser than air and 60 times as viscous.
The University of Montana, Missoula, graduate student Anthony Lapsansky and colleagues collected nine starlings to investigate how birds could first get into the water.Sturnus vulgaris), they choose 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, 3,500-liter tank. “I thought they would slam across the water surface to get out,” he recalls. “But they kept flying.”
Therefore, the researchers decided to immerse the birds in the water and simulate a fall from the sky. To their surprise, the starlings swam just as well as diving birds (see video above), Lapsansky reported this week at the Society for Integrative and Comparative Biology’s virtual annual meeting. They gracefully move their wings up and down as they accelerate to the surface.
The stroke of the starling is strikingly similar to that of parrot divers, murres and other divers. Until now, most researchers have assumed that the parrot’s slow stroke and bent wings (which pick up like helicopter blades to propel the bird through the water) are unique adaptations to underwater life. But these ‘unadapted’ starlings have also done so, suggesting that the behavior is innate or that it is the result of how the density of water limits how they can move, Lapsansky says.
Fish agrees. The fact that starlings immediately bend their wings suggests that the birds should do so to overcome the increased density of the water and to prevent their shoulder joint from being overemphasized. They already know how to bend their wings to get around obstacles. Just like parrot divers, the starlings used their feet to change direction so that they shared a different trait with diving birds.
Lapsansky and his colleagues also found another non-swimming species, the house sparrow (Passer domesticus), swim just as well as starlings. The bottom line, he says, is that it might not take as long as scientists thought for the transition from air to water; probably many birds winged it.