One of the most iconic birds in the United States is the northern cardinal, a crested songbird known for its scarlet-red—and on the rare occasion, yellow—color. Its hue makes the bird almost impossible to misidentify, but looks can be deceiving. A new study suggests the bird may have diverged into several different species that can only be told apart by their calls and genetic profile, according to a paper published in the journal Ecology and Evolution.
For the study, researchers took advantage of naturally occurring experiment, reports Ryan F. Mandelbaum at Gizmodo. In the southwest, a population of cardinals in the Sonoran desert, which includes the southwest corner of Arizona and southern California, is separated by about 120 miles of high plains from another population in the Chihuahuan Desert, which includes west Texas, New Mexico, the southeast corner of Arizona and eastern Mexico. The team first looked at the birds’ DNA and found that the two populations have been isolated from one another for 500,000 to 1 million years, which could be long enough for the process of speciation—the biological term for the formation of a new species—to begin.
They then listened to their songs, which are a crucial differentiator in the bird world. Each species of birds has its own repertoire of songs, calls and warning shouts. Over wide geographical areas, birds of the same species can develop regional dialects or variations. But if the birds are separated from one another for long enough, they will develop their own songs and calls that aren’t recognized by other birds of the same feather, a big step toward speciation.
For these desert cardinals, that seems to be the case. At 128 different sites, the team played various male cardinal songs, including some from the immediate neighborhood, others from the same desert but farther away, songs from the distant desert population and a cactus wren song as a control. They found that the Sonoran cardinals preferred Sonoran songs and the Chihuahuan cardinals preferred Chihuahuan songs, but when they heard songs from the other population, the birds ignored the tune.
“We saw that the birds are really aggressive to songs by their next-door neighbors, as you would expect, but once there is enough distance between them, they don't understand the songs anymore,” lead author Kaiya Provost of the American Museum of Natural History’s Richard Gilder Graduate School says in a press release. “It’s like if you speak Portuguese in Portugal, you can probably understand Spanish, and you might understand French, but if you keep going further and further away, eventually you’ll hit German or Arabic—languages that are unfamiliar, that you can't parse.”
While the Sonoran birds also ignored songs from distant birds in the same desert, the Chihuahuan cardinals reacted aggressively to the songs of their cousins, meaning there are some recognizable elements across the Chihuahuan calls that doesn’t appear in the Sonoran calls.
This isn’t the first indication that there may be multiple northern cardinal species lurking around our backyards. Mandelbaum at Gizmodo reports that in 2015, an independent researcher submitted a proposal to split the species into six based on the most up to date research to the American Ornithological Union, the body that determines such things. They declined, but it’s not a crazy idea.
In recent years, genetic data has revealed that many birds we thought were one giant population covering a continent are actually distinct populations. One study in 2016 suggested that once we sort out all the DNA and other distinct attributes like calls, there are probably 18,000 bird species on Earth, almost twice the current estimate. That work is already changing the bird life in our own backyards. In 2017, genetic testing helped identify a new species of crossbill that lives in just 26 square miles of Idaho. In 2016, the scrub jay of the southwest was split into two separate species, the California scrub-jay and Woodhouse’s scrub-jay. Similar splits are happening in bird taxonomy across the world every year to the delight and chagrin of serious birdwatchers.
So, what does this study mean for the cardinals? Not much, really. Whatever ornithologists decide to call them, the red birds will keep on singing whatever song is in their little birdy hearts.