Across North America, hundreds of bird species waste time and energy raising chicks that aren’t their own. They’re the victims of a “brood parasite” called the cowbird, which adds its own egg to their clutch, tricking another species into raising its offspring. One target, the yellow warbler, has a special call to warn egg-warming females when cowbirds are casing the area. Now, researchers have found the females act on that warning 1 day later—suggesting their long-term memories might be much better than thought.
“It’s a very sophisticated and subtle behavioral response,” says Erick Greene, a behavioral ecologist at the University of Montana, Missoula, who was not involved in the study. “Am I surprised? I guess I’m more in awe. It’s pretty dang cool.”
Birds have been dazzling scientists with their intellects for decades. Western scrub jays, for instance, can remember where they’ve stored food for the winter—and can even keep track of when it will spoil. There’s evidence that other birds might have a similarly impressive ability to remember certain meaningful calls.
“Animals are smart in the context in which they need to be smart,” says Mark Hauber, an animal behavior researcher at the University of Illinois, Urbana-Champaign (UIUC), and the Institute of Advanced Studies in Berlin, who co-authored the new study. He wanted to see whether yellow warblers had the capacity to remember their own important warning call known as a seet.
The birds make the staccato sound of this call only when a cowbird is near. When yellow warbler females hear it, they go back to their nests and sit tight. (It could just as well be called a “seat” call.) But it’s been unclear whether they still remember the warning in the morning.
So a UIUC team found 27 yellow warbler nests near campus and exposed females to either silence, or one of two sounds: a recording of a seet call or a recording of a generic warning—used for predators or competition—called a chip for 10 minutes. The next morning, the researchers observed the birds for 80 minutes: 20 minutes before sunrise and 60 minutes after, when the cowbirds are at their most active.
“These birds are really hard to see when there’s hardly any light out,” says Shelby Lawson, a behavioral ecologist at UIUC who led the study. “You basically stare at your binoculars for a solid hour because you don’t want to miss anything.” To be safe, the researchers also placed temperature sensors in the nests to detect when a bird was present.
They found the warblers left their nests less often after hearing a seet call than if they had heard no warning, the team reported last month in Biology Letters. The chip call didn’t seem to have any impact on how often they left the nest. “Sixteen hours after the experiment, the birds are still behaving as if there’s a cowbird threat,” Hauber says. “It allows us to think that these kinds of signals carry long-term meaning.”
Of more than 200 species targeted by cowbirds, yellow warblers are the only so far known to have developed a warning call tailored to cowbirds. This research shows the warblers can take it a step further, Hauber says, storing the knowledge passed on by the call using something resembling the scrub jays’ impressive long-term memory.
Lawson hopes to follow up by scanning the birds’ brains while playing various calls, to better understand how the information is processed. Does the same part of the brain light up when the birds hear the call of a cowbird as when they hear a seet call, for instance?
“These are animals that have the stepping stones of language,” Lawson says. “That must require some kind of higher order stuff going on in the brain, but right now we don’t know what’s going on in there at all.”