The northern cardinal is one of North America's most familiar songbirds. From the scarlet feathers and pointed crest of males to the rich, rhythmic songs of both sexes, it's an unmistakable icon of countless American forests, parks and backyards.

And as a new study demonstrates, northern cardinals are much more than just scenery and a soundtrack. As part of eastern North America's native biodiversity, they can also play a key role in keeping ecosystems — including humans — healthy.

That's according to new research from Atlanta, where a team of scientists wanted to figure out why more people don't get sick with West Nile virus (WNV). The mosquito-borne virus is zoonotic, meaning it can be spread between humans and other animals by a "bridge vector," a role played by Culex mosquitoes for WNV.

Since WNV was introduced to the U.S. in 1999, it has become the country's most common zoonotic disease carried by mosquitoes, causing more than 780,000 infections and 1,700 deaths. But for some reason, the virus sickens people in some areas more than others. It's abundant in both Georgia and Illinois, for example, showing up in nearly 30 percent of birds tested in Atlanta, compared with 18.5 percent in Chicago. Yet only 330 human cases have been reported throughout Georgia since 2001, while Illinois has seen 2,088 human cases since 2002.

Female cardinals build nests out of twigs in dense thickets or low, leafy trees. (Photo: Steve Byland/Shutterstock)

"When West Nile virus first arrived in the United States, we expected more transmission to humans in the South, because the South has a longer transmission season and the Culex mosquitos are common," says senior author Uriel Kitron, chair of environmental sciences at Emory University, in a statement. "But even though evidence shows high rates of the virus circulating in local bird populations, there is little West Nile virus in humans in Atlanta and the Southeast in general."

The reason for that difference has remained a mystery for years, prompting a three-year study by a team of scientists from Emory, the University of Georgia, the Georgia Department of Transportation and Texas A & M University. They collected mosquitoes and birds from various sites across Atlanta, tested them for WNV, and analyzed DNA from their blood meals to learn which birds they'd been biting.

"We found that the mosquitoes feed on American robins a lot from May to mid-July," says lead author Rebecca Levine, a former Emory Ph.D. student now working at the U.S. Centers for Disease Control and Prevention (CDC). "But for some unknown reason, in mid-July, during the critical time when the West Nile virus infection rate in mosquitos starts going up, they switch to feeding primarily on cardinals."

The benefits of bird biodiversity

American robins are considered 'super spreaders' of West Nile virus, but that effect may be balanced out by northern cardinals, which can reportedly help suppress outbreaks. (Photo: Stubblefield Photography/Shutterstock)

Previous research has shown American robins act as "super spreaders" of WNV in some cities like Chicago, Levine adds. Something about their blood creates a favorable environment for WNV, so the virus amplifies wildly once a robin is infected, meaning the birds can more efficiently pass it to new mosquitoes when bitten.

But cardinals have the opposite effect. Their blood is like an abyss for WNV, leading the researchers to describe the birds as "super suppressors" of the virus.

"You can think of the cardinals like a 'sink,' and West Nile virus like water draining out of that sink," Levine says. "The cardinals are absorbing the transmission of the virus and not usually passing it on." Cardinals seem to be the top suppressors of WNV, the study found, but similar effects are seen in birds from the mimid family — namely mockingbirds, brown thrashers and gray catbirds, all of which are common in Atlanta.

A city in a forest

Atlanta is known as 'the city in a forest' due to unusually high tree cover for a city its size, including old-growth forests like this one along the Chattahoochee River. (Photo: Nickolay Khoroshkov/Shutterstock)

These birds have all adapted to live among humans in cities, but they still need certain habitat features to thrive. Cardinals nest in dense thickets or low trees with lots of leaf cover, for example, and need a variety of seeds, fruits and insects to eat. And while they can't pinpoint the exact reason, Levine and her co-authors found fewer WNV-infected birds in certain parts of Atlanta: patches of old-growth forest.

Atlanta is nicknamed "the city in a forest," and for good reason: It's one of just seven U.S. cities with a high population density — more than 386 people per square kilometer — that still has urban tree cover of at least 40 percent. Chicago, by comparison, retains only 11 percent tree cover.

"With the extensive tree cover creating a unique feature of the urban landscape in Atlanta," the researchers write, "we also wanted to investigate how the effect of different urban microhabitats with differing degrees of tree cover might impact the ecology and epidemiology in the area." They found significantly fewer avian WNV infections at old-growth forest sites in Atlanta compared with secondary forests, even though the rate of infections in mosquitoes was similar in both forest types.

"These are really complex ecosystems, so we cannot single out the specific reasons for these findings," Levine says. "They suggest that there is something unique about these old-growth forests and how they affect avian systems in Atlanta.

"This finding suggests that old-growth forests may be an important part of an urban landscape," she adds, "not just because of the natural beauty of ancient trees, but because these habitats may also be a means of reducing transmission of some mosquito-borne diseases."

More research is needed to reveal why cardinals and primary forests have this effect on WNV, the researchers say, and to understand why mosquitoes switch from biting robins to cardinals in mid-July. But if such a familiar bird can offer an ecological benefit like this, it's hard not to wonder what other undiscovered perks lurk in old-growth forest fragments across North America — and for how much longer.