Why the Sparrow's Beak Is an Evolutionary Puzzle

For biologists, new findings about bird bills yield additional mysteries: an Object Lesson.

Inside Russell Greenberg’s office, just past the east entrance of the Smithsonian’s National Zoo in Washington, D.C., were three large, colorful photographs, each a portrait of a sparrow, each in near-profile. A friend and fellow researcher snapped the photos years ago in Mississippi’s swampland. Greenberg enlarged them and hung them from the wall above his desk. Like the sparrows, Greenberg has also been caught on film in the field, crouching, stooping, or pointing to something in the underbrush or vegetation, most likely a sparrow or its habitat.

Greenberg, who passed away two years ago at age 60, was an evolutionary biologist and the director of the Smithsonian Migratory Bird Center. He had been following sparrows around for nearly 20 years, and had come to know all about where these birds live, what they do, and how they behave. In fact, one might say, and Greenberg did say, that he was obsessed with them. In his last years, he was especially obsessed with their anatomy, and why sparrows, the males in particular, have bills that are nearly one-and-a-half times larger than their female counterparts, a peculiarity that scientists have overlooked until recently. He spent nearly a decade trying to figure out what’s behind the size discrepancy.

Greenberg suspected the explanation lies mainly in the bill’s ability to dissipate heat without losing water in a hot, dry climate—an explanation that’s a notable departure from previous hypotheses, which have long attributed sexual dimorphism in bill size to foraging and diet. The origin of this idea dates back to 1835, when Charles Darwin observed striking physical variation in the bills of Galápagos finches. Still, if heat dissipation affects bill size as Greenberg believed, he wondered why the males’ bills are so much larger than the females’ bills.

Greenberg said he noticed the size variation 25 years ago “when we first started playing around” with the coastal-marsh subspecies of swamp sparrow. “There was something about salt marshes,” he said.

Initially, Greenberg couldn’t be certain what that something was. But he kept thinking about research showing that toucan’s bills help them regulate their body temperature. In a 2009 paper published in the journal Science, researchers described how toucans can control blood flow to their bills, in essence turning them into transient thermal radiators. The larger the bill, the more surface area is available for cooling. Toucans’ bills are a whopping one-third of their body length. Greenberg came to suspect that sparrows might, too, be using their bills to cool themselves—salt marshes are typically hot, sunny, and windy, and shade and freshwater are limited.

So, Greenberg began measuring bill size in several species of salt-marsh sparrows that live, or had lived, in salt marshes throughout U.S. coastal regions, 1,500 species in all. “I measured them all personally,” he said. “Eighty percent of my research is done on dead birds,” many of which were collected en masse by ornithologists in the late 19th through mid 20th centuries. “It wasn’t just stamp collecting, which is the beauty of this whole thing,” Greenberg said. “They were collecting what we call series. A series is when you get a bunch of birds from the same place so you can look at variation.”

Which means that when Greenberg wasn’t in the field measuring bird bills and collecting data, he was spending afternoons at the Museum of Natural History, poring over its sparrow series. His analysis of the series confirmed that saltmarsh sparrows do indeed sport bigger bills, with roughly 40 percent more surface area than other sparrow species.

“Yes, there is a very strong tendency for saltmarsh sparrows to evolve bigger bills that are sexually dimorphic,” said Greenberg. “In other words, the bills of the species are bigger, but in particular the males’ bills are bigger. And sexual dimorphism in bill size is not common in sparrows. But the saltmarsh sparrows really stand out as having sexual-dimorphic bills.”

The bulk of Greenberg’s work took place in his office, or that of his collaborator, the  post-doctoral fellow Ray Danner. Together, they called on statistical analysis, graphs, and charts to make sense of their measurements and data. But it’s their observations in the field that made them more confident about their hypothesis.

Greenberg said females are taking care of offspring and foraging for food. They fly less, walk more, and stay low, where it’s shadier and cooler. “The males have a different thermal environment than females because of their sex roles,” he said. “And in birds, in a territorial male, his job is to be out there as much possible. Up. Exposed. Singing as long as possible. A successful male is going to have to be able to withstand things like solar radiation without too much water loss.” So the bills’ ability to radiate heat makes them especially useful to birds living in open habitats—like salt marshes—with little shade, freshwater, and convective winds, which help transfer heat away from the bill.

Greenberg and Danner noticed that the outer surface of males’ bills, known as the rhamphotheca, grows more quickly than females’. The rhamphotheca consists of keratin, a fibrous protein also found in human hair and nails, and grows and wears much like human nails do. Wear from foraging and from use in combat keeps the bill’s size in check. Having a larger rhamphotheca than a rival’s may pay off for a male during duels for territory, food, and females. Females may prefer mating with males with bigger bills because it’s a sign of fitness.

Despite these explanations, Danner and Greenberg still suspected that thermoregulation is the chief driving force behind this disparity. “That’s kind of our favorite hypothesis,” said Greenberg.

“We’re capable of having multiple, competing hypotheses,” said Danner. “It probably more accurately reflects the richness of natural selection. And often times it’s oversimplified. There are probably a lot of traits out there that are used for multiple things, and they’re selected by multiple things. But we don’t usually think about that.”

“We think of the bill as a tool, but even when you design a tool you have to think about things other than the job it’s doing,” said Greenberg. “For evolutionary biologists, the bill is iconic.”