In the summer of 2023, Jason Love, the associate director of the Highlands Biological Station in North Carolina, was about to set off on a jaunt along the Appalachian Trail. He was going Batpacking—backpacking, while also collecting data on the abundance of bats—and was strapping on gear in a gravelly parking lot. Love suddenly noticed a streak of red against white: a northern cardinal flapping furiously around the side-view mirror of a white Ford Escape SUV, around 30 feet away. "Oh, that's funny," Love remembered thinking. "The cardinal's attacking his reflection."
This behavior is fairly common for male cardinals, which frequently display aggressive behavior to defend their territory from other cardinals and intruders. Love had previously seen the birds attack his own car. "Generally when that happens, and other birds do it too, I get a plastic bag and cover the mirror, and that seems to solve it," he said. (He recommends this solution for anyone tired of getting bird poop on their car windows.)
As Love's fellow batpackers packed up their stuff in the parking lot, he watched the agitated bird through his binoculars. Suddenly, the bird switched tactics: picking up a pebble from the ground with his beak and smashing it into the side-view. "You could hear it hit the mirror," Love said. He turned to one of the batpackers, Romulus Stanek, then a student at North Carolina State University, who had a camera on hand. "How good is that camera?" Love asked Stanek. "Can it record video?"
Stanek filmed the cardinal for just under two minutes (you can watch the whole video here) and captured the bird striking the mirror four times with three pebbles, each with an audible tap. On the third try, the cardinal's pebble settled on the window ledge, where the bird retrieved it for a fourth and final attack. Love, Stanek, and Reagan Jarrett recently published their observation of the cardinal's pebble strikes in a paper in The Wilson Journal of Ornithology.
Love and his co-authors could not linger in the parking lot. But they resolved to write up the paper as a natural history report of tool use in northern cardinals, which has never been described before in scientific literature. When cardinals fight each other, they grapple with their feet or chomp at each other with their beaks; pebbles were not previously known to be in their arsenal. But in these fights, "usually there's a winner and a loser," Love said. "So whoever is the dominant would have chased it off and would have had no need for a pebble."
Plenty of papers have described aggressive tool use in other birds: a 1988 paper described a raven tearing twigs from a pine tree to lob at people who climbed near its nest, a sulphur-crested cockatoo dropping leafy branches on some bat hawks, and Australian brushturkeys throwing rocks towards intrusive lizards. But they only found one example of a bird actually holding on to an object during an attack rather than dropping the object on a target. This observation occurred when a Steller's jay ripped off a branch to attack a crow, which in turn picked up the same stick and lunged at the jay.
When Love first submitted the paper, peer reviewers asked if he'd looked to see if to see if other birds had been recorded using stones in this way. So Love went down a rabbit hole on YouTube, where he found a video of a white-winged chough striking a car windshield with a pebble and a video of a common raven dropping a seriously hefty stone on someone's dog. But the authors note that white-winged choughs use stones to crack open oyster shells, suggesting that the windshield attack may have sprung from curiosity rather than aggression. The raven, however, definitely dropped that rock with intent.
Parrots and corvids, which include crows, ravens, jays, and magpies, are known for their intelligence and tool use. The humble northern cardinal is not as renowned for for its intelligence. But cardinals do practice one notable form of tool use: anting, in which they place ants on their body so that the insects secrete formic acid to drive away feather mites and other parasites. Speculating about tool use, Love said that it's "looking like more animals do this than we're led on to believe."
The owner of the Ford Escape was not present for the cardinal's attacks. "I almost wanted to write a note," to explain why the windshield was cracked, Love said. "I would have loved to stay a bit longer and see if any other cardinals had picked up this behavior," he added. But batpacking called, and so he left the cardinal to defeat his own worst enemy, pebble by pebble.
