When Yellow Warblers Warn of Brood Parasites, Red-winged Blackbirds Listen
A new study found that the bird’s distinct warning call for Brown-headed Cowbirds might also benefit eavesdropping neighbors.
Spring is here and soon Yellow Warblers will begin arriving from their wintering grounds in Central and South America. Singing their sweet sweet sweet, I’m so sweet tune, these vibrant migrants breathe new life into moist scrub and wetland habitatacross much of the U.S.
While the Yellow Warbler is best known for its cheery song, the birds also produce a seet call that many birders mightrecognize. has shown that these calls are distinct from the bird’s alarm call for predators such as Blue Jays, which prey on their eggs, and are specifically meant to warn against Brown-headed Cowbirds, brood parasites that often target Yellow Warbler nests. When a female hears the call, sherushes back to the nest to prevent the cowbird from layingan egg.
Now, according to published last month in Communications Biology, it turns out another species might also benefit from these seet calls: Red-winged Blackbirds who nest near Yellow Warblers. “We’ve known about the seet call of the Yellow Warbler,” says Mark Hauber, a behavioral ecologist at the University of Illinois at Urbana-Champaign and co-author of the study. “What we didn’t know was that another species understands the seet call and considers it to be a danger to itself, too.”
Although cowbirds trick more than 200 bird species into raising their young, only Yellow Warblers produce a specific warning call that signals a cowbird’s presence. And though research has shown that several species, including nuthatches and hornbills, eavesdropon their neighbors’ conversations to gather intel on predators or good feeding spots, this is the first report of one species recognizing another'sbrood parasite warning.This abilitymight helpRed-wingeds in their owndefenses against cowbirds, the researchers say.
Hauber and his colleagues conducted their research in central Illinois, where breeding Yellow Warblers, Red-winged Blackbirds, and Brown-headed Cowbirds overlap between April and June. For the first experiment, they placed speakers innesting Yellow Warbler territory.Using playback of five different bird recordings—the warbler’s seet,the warbler’s general chip alarm call, cowbird calls,predatory Blue Jay calls, and a non-threatening Wood Thrush call—the researchers found that Red-wingeds appeared near thespeaker while raising their own alarm calls in response to almost half of theseet calls. Other birds showed up only a couple of times forthe seet calls.Meanwhile, theRed-wingedsturned up far less frequently for all theother calls.
For Shelby Lawson, a doctoral candidate in evolutionary ecology at the University of Illinois at Urbana-Champaign and lead author of the study, the more frequent appearances by the Red-winged Blackbirds made it clear that they had an ear for the warning calls.To further test out this idea, her team then set up another playback experiment in Red-winged nesting territory to see if the birds expressedsimilar signs of defense and aggression when hearing the seet calls, cowbird calls, Blue Jay calls, and the calls of other Red-winged Blackbirds, which can mean an intruder.
The researchers found that Red-winged males and egg-laying or incubating females almost immediately stopped their normal activity and sounded alarm calls or approached the playback speakersin response to Yellow Warbler seet calls and cowbird calls. But Red-wingeds also exhibited similar or slightly more aggressive behavior when the speakers playedBlue Jay calls and otherRed-wing calls, respectively. This indicated to the teamthat the Red-wingeds took the Yellow Warbler’s warning call to be as much of a concern as other common threats like predators or competition.
“It is super clear that they [Red-wingeds] understand the seet call to be some sort of alarm call or warning,” Lawson says, and “other birds aren’t responding similarly to these calls.”
What’s still not clear, however, is if the Red-wingeds take the seet calls to specifically mean a cowbird is present. To find that out, Lawson’s team will need to repeat the same experiment when the chicks hatch. At that point, cowbirds would no longer be a threat, and theRed-wingeds shouldn't react to the cowbird orseet calls.
Whether the Red-winged Blackbirds'eavesdropping skills actually translate to lowered brood parasitism also remains an unknown. Steve Rothstein, a behavioralecologist at the University of Santa Barbara who wasn’t involved in the study, thinks that the benefits, if any,may be small. “You could make it more difficult for cowbirds to monitor the host nest, but cowbirds are often able to overcome this host aggression,” he says. In other words, if a cowbird wants to sneak into a nest, it usually finds a way.
Although questions linger, the connection between Red-winged Blackbirds and the YellowWarbler’s seet call appears clear.So if you get the chance this spring towatch a Yellow Warbler flitting about, pay extra attention to its calls and an eye out for nearbyRed-wingeds.Who knows, you might get to witness a cowbird chase.