Since West Nile Virus was first detected in the United States in 1999, dead birds have become red flags for scientists tracking the virus. Though West Nile is spread by mosquitoes, many bird species act as carriers for the virus, hosting it until it can be transmitted to other animals by way of the next bug bite. Now, a new study published in the American Journal of Tropical Medicine and Hygiene suggests that some bird species​ might have another use when it comes to West Nile—they may actually help to stem the spread of the virus.

Scientists studying the virus’ spread throughout the U.S. have long been puzzled by how some cities, like Atlanta, Georgia, seem to have few human cases despite the virus being plentiful in the area. While the majority of people exposed to West Nile Virus don't experience any symptoms, for 20 to 30 percent of the population, the virus can cause flu-like symptoms and more severe neurological illnesses like meningitis and encephalitis.

Even though researchers have found that nearly a third of the birds in the Atlanta area have been exposed to the disease, Georgia’s first human case was only identified in 2001. Meanwhile, other cities with much lower rates of birds infected with West Nile, like Chicago, have reported cases of people being infected with the virus for years. Not only that, but Atlanta is home to a fairly large robin population—a species known for being an efficient “super-spreader” for West Nile.

“Robins do a very good job at amplifying the virus inside of them, but they don’t get sick and die,” Rebecca Levine, an epidemiologist and entomologist at the U.S. Centers for Disease Control who led the research, tells Smithsonian.com. “Something that dies from [West Nile Virus] has less chance to transmit it.”

Some birds like crows and blue jays are extremely susceptible to the virus (hence, the ominous die-offs), but others, like robins, act more as reservoirs for the virus to thrive until the next mosquito bite. However, while researching Atlanta’s robins for her doctoral studies at Emory University, Levine found that local cardinals were balancing out the robins by acting as “super-suppressors.”

“For whatever physiological reason…[robins] get a lot more virus in their blood than a cardinal does,” Levine says. “ A cardinal gets just about on the threshold of what it takes to transmit the virus.”

Levine says the mosquitoes responsible for transmitting the West Nile Virus like feeding on robins and cardinals about the same, but for some reason, robins are like little viral factories. Meanwhile, due to some fluke of biology, West Nile doesn’t reproduce as much in cardinals, making them less efficient carriers for the disease.

What makes this all so curious is that Levine and her colleagues found that while mosquitoes in Atlanta love to feed on robins earlier in the year, they switch over to a mostly cardinal diet around mid-July—right when the virus should be reaching the point where it spills over into the human population.

“If the mosquitoes had kept feeding on robins during that time, then we might see something very different,” Levine says. “The mosquitoes, right at the time when they are becoming more infectious, switch to feeding on this less competent host.”

Levine still isn’t sure why the mosquitoes have a change of appetite around this time, but it could have important ramifications for how officials deal with controlling the disease. By preserving the local pockets of old-growth forests where Georgia’s cardinals thrive, the state could help protect the bird populations that shield their human neighbors from the West Nile Virus. At the same time, researchers in other regions of the country might now be able to identify local super-suppressors that could prevent their own populace from exposure. While more research needs to be done, it appears that these little red birds are doing Georgia a world of good.

Editor's Note, August 10, 2016: The original title of this story incorrectly wrote that robins, not cardinals could help stem the spread of West Nile Virus. Additionally, the first case of West Nile in Georgia was detected in 2001. We regret the errors.