The first parasite Mark Hauber studied was a duck.
Specifically, it was a Redhead, one of around 100 bird species worldwide with an unusual method of raising their young: Known as brood parasites, they lay their eggs in other species’ nests, gambling that the host parents won’t reject the eggs or hatchlings and will instead raise the chicks as if their own.
This was in the 1990s, when Hauber was an undergraduate at Yale University. He was helping to run a Smithsonian Institution study to figure out how these parasitic ducklings knew which species they belonged to, despite not being raised by their parents. The vast majority of bird species imprint on their parents from birth. But the one percent that are brood parasites don’t use the standard social learning cues to recognize their own species. How do they do it? Hauber wondered. “That’s a really good question for all kinds of parasitic birds,” he says. “Not just ducklings, but cowbirds, cuckoos, indigobirds, honeyguides.”
“How do you know what species you are?”
That study found that young Redhead males raised by Canvasbacks, the primary host for Redhead parasitism, made “mistakes” about their identity, courting female Canvasbacks instead of their own species. The finding suggested that even brood parasites must learn what species they belong to, rather than knowing it intuitively. For Hauber, now a comparative psychologist at The City University of New York (CUNY) Graduate Center, this clarifying moment would shape his career.
Ever since then, he says he has been fascinated by a basic question: “How do you know what species you are?” In the process, he’s become one of the go-to experts on Brown-headed Cowbirds, widespread blackbirds that are North America’s most common brood parasite. Given that parasitic birds inevitably take a toll on their hosts—often removing the host’s own eggs and competing with her nestlings for food—people often judge them as “bad” and may even want to harm them to protect host species. But Hauber’s research paints a more sympathetic picture. Cowbird chicks don’t push other nestlings out of the nest, for example, and even seem to benefit from having nestmates when there’s enough food to go around.
Besides, Hauber tries to avoid applying a human-moral lens to birds that have survived and reproduced for at least 3 million years. Foisting their young on other species may seem “lazy” from an anthropomorphic perspective, but it’s a strategy that works: By avoiding the energy-intensive work of making a nest and parenting, wild cowbirds are capable of pumping out up to 50 eggs a season, compared to the 4 to 5 eggs per clutch that other birds typically lay. Hauber and his colleagues hope that sharing their creative and far-reaching studies on brood parasites will inspire new appreciation for the diversity of all birdlife. “Cowbirds represent really quite a unique type of that diversity,” Hauber says. “It represents something that is worth the study, worth the patience.”
More such study could be important, because like many other common birds, Brown-headed Cowbird populations are showing signs for concern. Their numbers have declined by 30 percent since the 1970s, likely due to losses of many of their grassland hosts, the insects they and their hosts eat, and suitable habitats. Targeted lethal control of cowbirds has helped to protect some vulnerable host species, Hauber says, but that doesn’t mean having fewer cowbirds on the landscape is a good thing. “We shouldn’t be the agents of natural selection ourselves,” he says. “We certainly have done enough damage to nature.”
Since his first stint with CUNY in 2009, Hauber’s research facility has been known as the “Cowbird Lab.” For years, he proudly marked the space with a glowing neon red sign. The light has since burned out. His enthusiasm for this misunderstood species has not.
‘He’s Done So Much’
Born in communist Hungary, Hauber recalls a favorite spring ritual as a boy: watching the annual return from Africa of Western House-Martins, plump swallows that build mud-cup nests under balconies and eaves. “I would grab my mother and say, ‘It’s time to look for the swallows,’” he says, “And we would walk the streets of this small town called Szolnok and look for the swallows repairing their nests and starting to take care of their young.” Entranced, he refused to consider any career but ornithology. His studies led him to apply for college in the United States—and to that fateful duck experiment at the Smithsonian.
He refused to consider any career but ornithology.
From that early experiment, through his Ph.D. at Cornell University and much of his globe-trotting research career, the behavioral ecologist obsessively explored the question of species recognition. Beginning with his graduate studies, cowbirds quickly became Hauber’s vessel for navigating those waters. He wanted to understand how young cowbirds grew up to socialize and eventually mate with others like themselves.
In an influential 2001 study, he discovered that six-day-old cowbird nestlings begged more after hearing recordings of cowbird chatter calls—a “password” that unlocked certain behaviors likely to help them recognize and thrive with their own species. Nearly two decades later, after Hauber had moved his lab to the University of Illinois Urbana-Champaign, he and colleagues uncovered that this password was the key to better song learning for young male cowbirds.
He also chipped away at the mystery of parasite identity in a 2000 experiment that involved dyeing young cowbirds’ feathers. This approach confirmed that a phenomenon known as the “armpit effect” is present in birds. The name refers to humans’ and other animals’ ability to tell relatives from others based on body odor. Hauber’s study highlighted a visual form of the armpit effect by demonstrating that cowbirds tune into their own feather colors and flock to birds with similar hues. “That’s the mechanism that we use to recognize our relatives using olfactory cues, but the birds use that for visual cues to recognize their own relatives,” says Hauber, who returned the Cowbird Lab to CUNY in 2023.
Beyond how a cowbird recognizes its own, there remained the question of how a young cowbird leaves its host family to join them. There, too, Hauber’s team has uncovered exciting discoveries. Prior to a 2015 study that he co-authored, he and other scientists believed that adult females must return and guide juveniles away from the nest. But the experiment revealed that young cowbirds suddenly disappear some nights from their foster parents to sleep by themselves. This completely unexpected finding “rocked” scientists’ understanding of how fledgling cowbirds might learn and become independent, says Mac Chamberlain, a Ph.D. student under Hauber at the University of Illinois Urbana-Champaign.
What’s most impressive about Hauber’s work on parasitism is its breadth and depth, says Mélanie Guigueno, a behavioral ecologist at McGill University in Canada who studies the complex cognitive abilities of cowbirds. It’s also his lab’s prolific way of working across disciplines: the chemistry of egg color, the physiology behind why hosts reject eggs, the optics of plumage iridescence. “Oh my gosh,” Guigueno says. “He’s done so much.”
Now far from Illinois’ fields and back in CUNY’s labs, Hauber’s curiosity is following a big new question: “What does it take to build a cowbird?” He wants to find out how the signature traits of brood parasites, such as better spatial memory for finding nests in the dark, evolved in species that parented earlier in their evolutionary history.
As part of that effort he’s searching for an egg-recognition center in the bird brain, much like humans have an area specializing in faces. “I think eggs for birds are what faces are for humans,” says Hauber, “I’m hoping to identify an area in the brain where birds say, ‘Eureka! This is an egg.’”
Three decades after first studying Brown-headed Cowbirds, Hauber still finds these deceptively simple-looking birds fascinating. “It’s what’s under the skin of the cowbird,” he says. “When it comes to their brain and behavior, they are anything but regular. I love that conundrum.”
