Like all immigrants, young male songbirds arriving in the United States have to make some quick decisions, beginning with finding the best place to build a nest. A new study reveals that youngsters make their choices after eavesdropping on the songs of their elders. The results add to a growing body of research indicating that birds' songs carry far more social information than scientists realize.
Finding the right nesting habitat is key to a songbird's reproductive success, says Matthew Betts, a landscape ecologist at Oregon State University, Corvallis, and the lead author of the study. "Even 50% of experienced males will lose their chicks to predators," he says, so it makes sense for birds to look for places with the most cover. Other studies have shown, however, that songbirds are drawn to areas where they see and hear their fellows nesting. Betts and colleagues suspected that house-hunting young male black-throated blue warblers (Dendroica caerulescens), who arrive in the States from Jamaica, cue in on the songs of their elders.
Successful warbler dads sing after their chicks have fledged, most likely to teach their songs to their offspring, says Betts. "But there could be another, unintentional message in their song: 'Hey, I've reproduced,' "--a clue that the older bird is sitting on prime nesting real estate.
To test that idea, Betts and his colleagues recorded males' songs at nests with fledglings in good habitat: a 90-year-old forest in New Hampshire's White Mountains. They then played the recordings at 18 sites with lousy warbler habitat: recently clear-cut forest. Young male warblers flying nearby heard the calls and apparently memorized the exact locations of the crummy locales. That's where they settled the next spring, after migrating to the Caribbean for the winter. As the team reports online this week in the Proceedings of the Royal Society B, the birds were four times more likely to follow the cues of the songs than to choose a site based on their own observations. Eighty-three percent of the 23 first-time breeding males moved right in. "We were very surprised," says Betts. "It was as if we'd attracted a spotted owl to a parking lot."
"While public information is increasingly acknowledged as important" to how animals make decisions, says Mikko Makkonen, an ecologist at the University of Jyväskylä in Finland, this is the first experimental study to show that the information can override what a bird sees. Indeed, notes Hanna Kokko, a behavioral ecologist at the University of Helsinki in Finland, once males have made their poor choices based on "hearsay," females follow them in. "It's a cascade of mistaken information," she says.