How Well Do We Really Know Cardinals?

Few birds are as well-known or beloved as the Northern Cardinal. The quintessential red (and sometimes yellow) species serves as an emblem for seven states and countless sports teams—yet much about it still remains a mystery, as shown by a recent Ecology and Evolution study on a pair of Southwestern populations.

The research, led by scientists at the American Museum of Natural History in New York, confirms that cardinals from the Sonoran and Chihuahuan deserts don’t interbreed, despite being separated by just 120 miles of plains habitat. While the Sonoran group is already considered a subspecies, these findings hint that it may need to be reclassified as its own species. To put it into perspective, that would mean that the Arizona Cardinals would rep a different bird than the St. Louis Cardinals.

“Everybody thinks, ‘It’s a cardinal, I know what a cardinal looks like,’ ” says lead author Kaiya Provost, a Ph.D. student at the museum’s Richard Gilder Graduate School. “But when you really start digging in you realize, ‘Oh, maybe they’re not what I thought they were.’ ”

Ornithologists like Provost have long been aware of a certain degree of cardinal diversity. The IOC World Bird List currently recognizes 19 Northern Cardinal subspecies, most of which were discovered over a century ago. What’s more, in a pair of genetic studies earlier this decade, Provost’s academic adviser Brian Smith identified six reproductively isolated cardinal populations across North America (four of which live exclusively in Mexico).

Smith’s findings propelled a 2014 proposal to split the Northern Cardinal into six species. But the American Ornithologists’ Union checklist committee, the leading local authority on bird taxonomy, rejected it unanimously, citing a lack of vocalization studies and other supporting data.

That’s where this new paper comes in. Through genetic comparisons crunched by complex computer models, Provost, Smith, and a third co-author, William Mauck, determined that the Sonoran population (~150,000) diverged from the Chihuahuan one (~700,000) about a million years ago. They found that some interbreeding may have taken place in the past, but likely not in the last tens of thousands of years.

Part of the reason could be the birds’ unique field marks. Unlike their eastern counterparts, Sonoran males have a pale brick-red look with a taller crest and less black on the face. They don’t sound similar either: Provost loosely describes the songs of the two populations to be as different as Portuguese and German. “The phrases they use have some overlap, but the way they put them together doesn’t match,” she says. (Listen to the “long-crested” clips here.)

To better learn how vocals fit into the cardinals’ breeding strategy, Provost and her team traveled to the Sonoran Desert near Portal, Arizona, and to the Chihuahuan Desert in Big Bend National Park, Texas, where they played four song recordings for dozens of territorial males: one of a neighboring cardinal, one of a non-neighboring cardinal from the same desert, one of a cardinal from the other desert, and one of a Cactus Wren (the experimental control).

In general, the males acted hyper aggressively with their perceived neighbors, but ignored rivals from the other desert. Previous studies show females reacting tepidly to songs from other populations; so, with that in mind, the authors concluded that any cardinal that survives the flight across the high plains—a treacherous stretch filled with “tumbleweeds, dust storms, and a lot of billboards,” Provost says—would still be unable to flirt and find a mate.

But don’t stamp the species’ divorce papers just yet. Kenn Kaufman, Audubon field editor and bird expert, cautions that the latest study is “suggestive but not overwhelmingly so.” He points out that during the experiments, the Sonoran cardinals ignored the songs of distant individuals from the same desert, even as the Chihuahuan cardinals didn’t. That means there could be more factors at play.

The study does have one definitive takeaway, though: that ever-cheaper and -convenient technologies are upending the avian family tree. “We’re starting to see a number of [splits among] species that are very similar looking, [but that] talk different and act different and don’t mate,” says Geoff LeBaron, Audubon’s Christmas Bird Count director. He points to the 2010 rejiggering of the Winter Wren as an example, and adds that Marsh Wrens, Warbling Vireos, and Eastern Meadowlarks might be among the next to go. Looks like the near-estranged Sonoran cardinals will be in good company.