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Mockingbirds in fickle climates sing fancier tunes

26.05.2009
Why are some birds simple singers and others vocal virtuosos? Researchers at the National Evolutionary Synthesis Center (NESCent), the Cornell Lab of Ornithology, and McGill University suspect that inconsistent climates may play a role.

A large-scale study of mockingbirds in diverse habitats reveals that species in more variable climes also sing more complex tunes. "As environments become more variable or unpredictable, song displays become more elaborate," said Carlos Botero, a postdoctoral researcher at NESCent in Durham, NC. NESCent is an NSF-funded collaborative research center operated by Duke University, the University of North Carolina at Chapel Hill, and North Carolina State University.

Local climate patterns are good indicators of how challenging life is in a given location, Botero said. "Survival and reproduction become more complicated when weather patterns are unpredictable because you don't know when food will be available or how long it will be around," he explains. What's more, the consequences of picking a mediocre mate are magnified in harsher climes.

"In really difficult or demanding environments you would expect females to be choosier," he said.

Male mockingbirds sing primarily to impress mates, said Botero. Superior singing skills are a cue that a male is a good catch. "Complexity of song display – how many song types a bird sings, how hard the songs are − is a good predictor of the quality of the individual," said Botero. "Males that sing more complex songs tend to carry fewer parasites, and have offspring that are more likely to survive."

Songbirds aren't born knowing their songs, however: they have to learn them over time. Since birdsong is a learned behavior, Botero and colleagues suspect that song-learning ability may also be a display of learning ability in general. The bird equivalent of sparkling conversation, complex songs may indicate which males have not only brawn, but also brainpower. "Birds that sing better are telling others, at least indirectly: Hey, I'm a good learner," said Botero.

More importantly, singing skills may be a sign that males are clever enough to cope with iffy environments. "Individuals that are more intelligent tend to be better able to compensate for the difficulties of unpredictable climates. For example, if some individuals are able to invent new foraging techniques, then they are going to be better at surviving harsh winters than the poor guys who only know one way to forage," Botero said. "The more intelligent you are, the more resourceful you are, and the more curve balls you're able to handle."

To see if there was a correlation between climate and song, Botero searched sound archives around the world and embarked on a solo tour of the southern hemisphere to record bird songs in the wild. Armed with supersensitive recording equipment, Botero trekked his way through desert, jungle, scree and scrub in search of mockingbirds in song. Botero's recordings − nearly 100 tracks from 29 mockingbird species − will enhance pre-existing sound archives by filling in gaps for species for which high-quality recordings weren't previously available.

Back in the States, Botero used computer programs to convert each sound recording − a medley of whistles, warbles, trills and twitters − into a sonogram, or sound graph. Like a musical score, the complex pattern of lines and streaks in a sonogram enables scientists to see and visually analyze sound.

Botero and colleagues then painstakingly analyzed each snippet of song and compared their patterns to a database of temperature and precipitation records. The researchers found that species subject to more variable and unpredictable climates had more elaborate song displays.

The connection between birdsong and climate is new and somewhat surprising, Botero explains. "We're connecting two dots that were far away before."

For Botero and his colleagues, the next step is to see whether this pattern holds true for other animals. By studying animal communication, Botero ultimately hopes to shed light on how language evolved in humans. "You can't help but wonder what is it about humans that made our vocal communication so incredibly complicated compared to other animals," Botero said.

"It has long been hypothesized that one reason why humans have such exaggerated displays – not just language, but music, art, and even math – is because females have selected for signals of intelligence," explains Botero.

"What we have now is a nice arena – outside of humans − where we can test these ideas and start understanding processes that are fundamentally important for our own species."

The team's findings were published online in the May 21 issue of the journal Current Biology.

Robin Smith | EurekAlert!
Further information:
http://www.duke.edu

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