Just like human children, young songbirds need to hear other birds to develop normal vocalizations. Unlike most sparrows, the diminutive grasshopper sparrow uses two different songs, which it memorizes in different ways. Researchers at Ohio State University's Borror Laboratory of Bioacoustics are working to find out how.
The grasshopper sparrow is a plain little bird. It lives in grasslands throughout much of the U.S., and thrives in reclaimed Ohio strip mines.
"They're mostly brown, they have tan and white, a little bit of yellow kind of near their eyes. Pretty subtle."
Jill Soha, a curator at the Borror Lab, is more interested in these birds' songs than their drab looks.
Sparrows have two main messages they transmit in song. They stake out territory, and they attract and bond with mates. Most sparrows use a single phrase for both purposes.
But male grasshopper sparrows learn two distinct song types one for territory, and one for mates. The simpler song is known as the buzz song.
"It's got a few introductory notes and then that buzzy note that's sustained and kind of grasshopper like," she said.(Song.) "That's that male-male territorial signal."
The other song serves a more romantic purpose.
"This second one which I call the warble song type sounds like this at full speed." (Song.) "So it's got a lot of individual notes that are pretty sharp and fast," Soha said.
For human ears to appreciate the detail of the warble song, it helps to slow it down. This is the same song at half speed. (Song.)
Here are the normal full-speed buzz and warble again. (Songs)
It's well known that songbirds need to hear examples of their own species singing to develop normal songs. But since grasshopper sparrows learn two songs, Soha wanted to know if they learn the songs differently. They might precisely imitate songs they hear, or they might improvise
"They could have to hear songs of adult males," she said. "Not necessarily their father, but some adult males, in order to get an idea of what they should sing but not necessarily imitate that exactly."
To find out, she reared young male sparrows under three different conditions. Each had his own cage, but some grew up without hearing any sparrow songs at all. Others heard recordings of buzz and warble songs from wild grasshopper sparrows. And the third group could see and hear live adult sparrows in adjacent cages.
As expected, the isolated birds never produced normal songs. Compared to the normal territorial buzz, (song,) their song was too raspy (song).
The tape-tutored birds made a normal buzz, but it was not a precise imitation of the recording. Rather, the birds improvised the details. (Song.)
In contrast to birds that heard only recordings, birds that learned from other birds did imitate their tutors. So, both hearing the buzz song and interacting with adult birds make a difference. This song imitation is typical of most sparrows that employ a single song type.
It's their second, more complicated warble song that sets grasshoppers sparrows apart. Again, isolated birds didn't produce a recognizable warble. This buzzy version was as close as they came. (Song.)
But tutored birds all improvised, whether they learned from a recording or from another bird. Their songs were normal warbles but they rearranged the individual notes in their own sequence.
Soha concludes that the sparrows normally learn the two songs in different ways.
"These two song types, they imitated one more closely, but the second song type they still kind of improvised," she sai. "So it seems that there are some effects of social interaction on whether they improvise or imitate, at least for one song type but not the other."
Soha says that a greater improvisation and individuality in the warble song could help female grasshopper sparrows recognize their mates. She presented her research at the annual meeting of the Animal Behavior Society in Burlington, Vermont.
Elsa Youngsteadt WOSU News