Across North America, hundreds of bird species are victims of the cowbird, which adds its own egg to other birds' nests, tricking another species into raising its offspring. One target, the yellow warbler (林莺) , has a special call to warn egg-warming females when cowbirds are approaching the area. Now, researchers have found the females act on that warning one day later一suggesting their long-term memories might be much better than thought.
"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 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 call only when a cowbird is near. When yellow warbler females hear it, they go back to their nests and sit tight. But it's been unclear whether they still remember the warning in the morning.
So the study team found 27 yellow warbler nests and exposed females to either silence, or one of two sounds: a recording of a seet call or a recording of a common 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.
They found the warblers left their nests less often after hearing a seet call than if they had heard no warning. The chip call didn't seem to have any impact on how often they left the nest. Sixteen hours after the experiment, the birds were still behaving as if there's a cowbird threat. It allowed us to think that these kinds of signals carried 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.
