A single courtship song from a male bird can rouse different responses depending on whether females have recently heard avian Pavarottis or the equivalent of tone-deaf singers in the shower.
Female Lincoln’s sparrows (Melospiza lincolnii) favor males that sing strenuous trills quickly, explains Keith Sockman of the University of North Carolina at Chapel Hill. But after a week in the lab listening to recordings of impressively fast trilling, female sparrows showed relatively modest interest in a new song with merely intermediate-speed flourishes. That same OK song, however, won more attention from females subjected to a week of slow trilling, Sockman and his colleagues report October 15 in Biology Letters.
In one sense, this bias in judging the songs sounds irrational, says coauthor Susan Lyons, also of UNC. “Decisions should depend on the current situation, not how the individuals arrived at the current situation.”
Yet in a world where conditions can vary from season to season, changeable standards may be “ecologically rational,” Lyons says. Sockman suggests that if females didn’t adjust their response to male song, some might fail to find a mate at all during a year of pathetic crooners.
Wonderful, terrible birdsongs
One of the Lincoln’s sparrow songs used in the experiments, with four trills, plays at the bird’s natural singing speed.
The sparrow’s song is tweaked to create a virtuoso version, with syllables making up each trill following each other 15 milliseconds faster than they did in the original.
This version represents a low-quality song, with trills slowed by insertion of an extra 15 milliseconds between the component syllables.
At the end of the experiment, novel songs such as this one were played to female birds, with trills of intermediate quality.
Credit: K. Sockman
Researchers have found similar distortions in judgments of the opposite sex’s attractiveness in fish (sticklebacks and mottled sculpins), zebra finches, fruit flies and field crickets, notes behavioral biologist Tim Fawcett of the University of Bristol in England. People aren’t immune. Male undergraduates were relatively harsh in judging the charms of an average woman after they had watched an episode of Charlie’s Angels.
What’s unusual about the new birdsong study, Fawcett says, is that the Sockman lab has already laid a foundation for connecting real-world variation to the bias based on contrasts. A previous sparrow paper showed that male song performance — including the average trill speed — can vary from one year to the next.
To check for biases based on song contrasts among Lincoln’s sparrows, Sockman and colleagues tweaked one aspect of performance, the trill speed, in 18 real sparrow songs. Adding 15 milliseconds between the basic components, or syllables, of recorded trills created nine pathetic serenades that lie within the range of what birds naturally hear at a study site in Molas Pass, Colo. Snipping out 15 seconds made flashy versions of the same songs.
Researchers played either the slow or fast songs intermittently for six hours each day and finally presented the 12 females in the study with a novel, intermediate-speed song. To judge a female’s enthusiasm for the new song, researchers monitored whether females moved near the broadcasting speaker. The females that had just spent a week hearing crummy songs spent roughly twice as much time near the speaker as did females finishing a week of great music.
The contrast with the previous week’s songs clearly has an effect, Fawcett says. However, this experiment can’t prove whether the contrast enhances or detracts from the reaction to the novel song. That would take comparison with a third group of birds listening to an intermediate-speed song in the prior week.
Contrasts and changing frames of reference matter to more than flirtations, says mathematical biologist James J. Anderson of the University of Washington in Seattle. He has studied how fish reactions to water speeds affect whether migrants cope with some dams. “In both birds and fish,” he says, “it seems that understanding behavior requires understanding the reference frame and how it changes over time.”