Populations of the Northern Cardinal can be found in the southwest United States and Mexico in two deserts, the Sonoran and the Chihuahuan.Winnu/Flickr
The bright-red Northern Cardinal is a familiar sight to many backyard bird-watchers across the United States. But a new study from Museum researchers that analyzed genetics and songs across different populations points to evidence that there may in fact be multiple species.
“ It is getting harder to argue that they are a single species.”
The research team studied Northern Cardinal populations in two deserts, which are separated by about 120 miles of high-elevation plains: the Sonoran Desert, which covers parts of Arizona, California, and Mexico, and the Chihuahuan Desert, which includes part of Texas, Arizona, New Mexico, and Mexico.
When the researchers analyzed DNA of the birds in these areas, they confirmed that the two populations have been separated for at least 500,000 years and possibly for as long as 1 million years—long enough for the process of speciation to take place. At the same time, the team also examined the song-related behavior of these populations.
“In general, songs are really important for describing and identifying birds,” says Kaiya Provost, a doctoral student in the American Museum of Natural History’s Richard Gilder Graduate School and lead author of the study, which was published this week in Ecology and Evolution. “Most studies assume that differences in song are important in the process that gives rise to new bird species. But looking at speciation using both genetics and behavior in wild birds can be really difficult. We went out to test both of these spheres of biology on wild desert birds to look at the full story.”
Songs play a crucial role in a bird’s ability to attract and impress a potential mate. If two birds can’t communicate with each other, they are less likely to breed. Over time, populations that don’t reproduce with each other will accumulate more and more genetic differences. As time goes on, these two processes can feed back into each other, leading the populations down the path of speciation.
To investigate, Provost and her collaborators—Brian Smith, an assistant curator in the Museum’s Department of Ornithology, and William Mauck III, a researcher at the New York Genome Center—created a bird song experiment that they played in eachdesert. Each audio series contained four recordings of male birds: neighboring cardinals, cardinals from the same desert but a distance away, cardinals from the adjacent desert, and a control recording of a Cactus Wren. (Click below to hear samples of two birdsongs recorded by researchers, the first in Arizona and the second just over the border with Texas.)
In the Sonoran Desert, male cardinals reacted to the recorded songs from neighboring birds with aggression—flying around looking for the “intruder” and singing loudly. Songs from birds living further away, both from within the same desert and from the adjacent one, were ignored.
“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,” Provost says. “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.”
In the Chihuahuan Desert, the cardinals also acted aggressively to songs from close neighbors. And, just like the Sonoran birds, they ignored songs from birds across the plains. But, in contrast to Sonoran cardinals, they were aggressive to songs from distant neighbors in the same desert.
“We’re not sure why there’s a difference, but you can think of it as these Chihuahuan birds singing in Portuguese and hearing songs in Spanish. It’s a little different but they still understand it, and they still think it’s an intruder,” Provost says. “There’s something that’s keeping those two groups of songs linked together.”
One of the major challenges taxonomists face is how to identify young species, or draw the line between species and populations. In the case of the Northern Cardinal, the researchers say there is mounting evidence that there are in fact multiple species in the United States.
“By combining behavioral experiments with genetic estimates of population history, we found corroborating evidence that the speciation process is well advanced,” Smith says. “It is getting harder to argue that they are a single species.”