Male and female cardinals recognize each other during breeding season not by their plumage, but by differences in their songs, a Columbia University researcher has discovered.
Both male and female cardinals learn their songs, a trait that is exceedingly rare in the animal kingdom, and is also found only in parrots and other avian mimics, dolphins and whales. Scientists study these species to discover more about how songs are learned, information that could show how human babies acquire speech -- a process that is still not fully understood.
By working with baby cardinals, Ayako Yamaguchi, postdoctoral research scientist at Columbia, found that male birds can learn songs from adult females, and females from males. But as the birds grow, their songs become different, probably as the result of hormonal changes.
"If they are learning to sing, how is it that all males sound one way, and all females sound a different way?" Dr. Yamaguchi asked. Her work appears in the August issue of Condor magazine.
Cardinals, which had previously ranged from Central America to Florida and Arizona, have become common in North America only in the last century, mainly because humans have provided food that allows the hardy bird to survive the winter. The male is bright red, the female brown with a pinkish orange head.
Ornithologists had never distinguished between male and female cardinal songs, since there is no distinction between the two to human ears and because most scientists assumed that female cardinals, like the females of most songbird species, don't sing at all. Dr. Yamaguchi was intrigued that cardinals sing only during breeding season from early spring to midsummer, and reasoned that, since the birds normally inhabit dense underbrush that hides them from view, their songs might provide the cue that alerts a bird to the presence of another bird.
She hid speakers in the brush at a field site in Arizona, and when she played back male songs, male cardinals attacked the location of the speaker; when she played back female songs, females responded with a song. Since there were only breeding pairs at the site, none attempted to pair with the speaker.
Armed with the knowledge that cardinals can distinguish gender by song, Dr. Yamaguchi undertook an exhaustive acoustic analysis of both male and female songs. Male birds tend to more accurately repeat a single stereotyped "twee, twee, twee," while the female song varies somewhat more, she found. Though both songs have the same pitch, the male song is also a purer tone, which may allow it to travel further in the forest; the female song has significantly louder second harmonics, giving it a nasal quality. Amplitude is difficult to capture in the field and depends on the position of the bird and the listener, Dr. Yamaguchi said, so it's not clear whether the male song is louder.
In an earlier study, she took blood samples from 30 young cardinals kept in a laboratory, and, as they developed, found that testosterone levels shot up in the males at about the time their songs became more stereotyped and less nasal. Dr. Yamaguchi says that young cardinals have a babbling phase similar to that in humans, that they then learn songs from older cardinals, and that in the juvenile phase the songs become more adult-sounding. It's likely that testosterone increases the weight of the muscle attached to the male's syrinx, or vocal organ, she adds, resulting in the male's different song.
The work was supported by the Chapman Fund of the American Museum of Natural History, the Sigma Xi Scientific Research Society and several Jastro Shields Grants from the University of California, Davis. The work forms part of Dr. Yamaguchi's doctoral dissertation at the University of California, Davis.
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