New scientific results have strengthened the case that the ubiquitous northern cardinal could be two species of bird or more.
Scientists most recently performed an analysis of cardinals on either side of a natural barrier that separates two populations of northern cardinals—those in the eastern half of the United States, and those around Arizona and to the south. Both genetic and behavioral differences seem to suggest that the boundary has led to the birds to evolve into different species.
“We look at the genetics and songs they’re singing and ask whether they’re different,” the study’s first author Kaiya Provost, a Ph.D student from the American Museum of Natural History, told Gizmodo. “To be able to say yes, they are different, is really exciting to me.”
The evidence begins with the bird’s songs. Song is an important trait that can help tease out the differences between species. Males use their songs to attract mates, and act aggressively against males from the same species in their territory—but not as aggressively against songs from different species. How either population of cardinals reacts to the other’s song can offer evidence to how genes are flowing between the group.
The researchers played cardinal songs for four trials at 128 different sites, 67 in the Sonoran desert and 61 in the Chihuahuan desert. The cardinals at the sites generally came much closer to the speakers when songs from the local population played than they did when songs from the other population did, according to the paper published recently in the journal Ecology and Evolution.
“They don’t sing the same songs,” said Provost. “That’s another piece of evidence that there isn’t one species—there might be more.”
Furthering the evidence, the researchers performed a genetic analysis of cardinals in either group. It demonstrated that the two cardinal populations likely diverged approximately a million years ago.
What’s going on? It seems that the two populations of northern cardinals are separated by the “Cochise filter barrier,” the region of habitat that separates the Chihuahuan and Sonoran deserts that prevents some but not all species from moving between them. The researchers found “no support” for genes currently flowing between the populations, though the two groups could have interbred some time in the past.
This isn’t the first inkling that the cardinal is more complicated than it appears. Back in 2015, an independent ornithologist issued a formal proposal to the American Ornithologists’ Union (AOU) to split the bird into six species, based on plenty of published research. The AOU rejected that proposal—but this new study strengthens the case.
The study has its caveats, explained Provost. There could be as many as six species of cardinal based on the evidence. Aside from that, the researchers only tested male, not female behavior—though unpublished evidence suggests that females prefer males who sing the local song .
It will be up to the AOU to decide whether or not the evidence warrants a new species for birders’ checklists or not. As for the birds, they probably don’t care either way.