Birds sing to communicate to each other. And mostly, it’s the males who do most of the singing. In Massachusetts, a biologist is studying the song of the Chestnut-sided Warbler to find out how and why the male attracts females. Nancy Cohen reports.
CURWOOD: Many songbirds live in the tropics, but head north to more temperate climes in the spring when it comes time for breeding. One reason: the eating's better up north as great volumes of insects and other bird food also emerge in the spring. But, as Nancy Cohen reports, it ain't the chow that the birds are singing about when they get here.
COHEN: Although it may seem that birds sing because they enjoy it, their intricate songs are actually a tool for communicating. In the world of birds, it's the males who do most of the singing. Dr. Bruce Byers, a biologist from the University of Massachusetts in Amherst, says they sing in contests with other males, and when they sing to females, as Byers translates it, they're broadcasting a kind of personal ad.
BYERS: "I'm better than the other males. I'm better than my neighbor. I'm stronger. I'm healthier. I have better genes for you to pass to your offspring. I'm strong, I'm sexy. Come mate with me."
COHEN: Byers is a quiet guy who's spent more than a decade studying the songs of one species, the small yellow capped, chestnut sided warbler. The bird's song is best described as, well, warbly.
BYERS: The notes kind of tumble out one after the other in kind of a watery fashion, almost.
COHEN: Byers has chosen to study the chestnut-sided warbler because of this song, the one males use to attract females. The song doesn't vary much from bird to bird, nor does it vary much between renditions sung by a single bird, which raises the question: what does a female listen for in these songs?
BYERS: It seems unlikely that females were basing their choice on, say, the repertoire or the complexity of these songs. They must have been basing it on something else.
COHEN: Byers' goal is to find out if there is something special about the songs of the successful males, the ones who have the highest number of offspring. To find the answer, he spends every spring going to work well before dawn.
[SOUNDS OF HIKING]
COHEN: It's June, about 4:00 in the morning. Byers is making his way down a muddy trail in the Savoy State Forest, about three hours west of Boston. His field site is on the edge of the woods. It's not exactly what you'd call pristine. It's a swath of cleared land that runs beneath towering electric power lines. The power poles don't seem to bother these birds. In fact, they build their nests right underneath them.
BYERS: It's Thursday, June 14th, at 4:43 a.m., recording a bird halfway between pole seven and pole eight.
COHEN: Byers spends the morning tromping up and down the hillside, tuning his ear to the birds and turning his tape recorder on and off and on again.
BYERS: Trying to get on this bird, halfway between eight and nine.
COHEN: Byers uses a parabolic microphone which juts out from the center of a large plastic dish. Any sound waves that hit the dish reflect back onto the microphone. This means he'll get a good recording without having to aim the mic precisely at the birds. This is what one of his recordings sounds like: [sound clip of bird song] This song, the chestnut sided warbler's mate attraction song, is transliterated in field guides this way:
BYERS: "Pleased, pleased, pleased, to MEET-cha." Or "Please, please, please, Miss BEECh-er." And to me it sounds like "Wheat, wheat, wheat, wheat, WHEAT-chu." Those last two syllables, the "wheat-chu" is louder than the introductory part of the song.
COHEN: After two breeding seasons, Byers has recorded hundreds of these mate attraction songs for each of the 56 male birds in this study. As he records, he identifies each singer by the multi-colored bands that he placed on the birds' legs earlier in the study. This allows Byers to create a profile of each male's singing style and correlate that with the bird's reproductive success. To find out which males had the most offspring, he takes blood samples and compares the DNA of each chick with that of both parents from every nest in his study.
By early September, the chestnut-sided warblers are migrating to Central America where they spend the winter, and Byers goes back to his lab, a drab cinder block room at the University of Massachusetts in Amherst.
[SOUND OF TYPING]
COHEN: Byers is taking a close look at the songs of the males with the most offspring-- a very close look. Here, he's measuring frequency.
BYERS: And now we have this-- a graph has appeared on the screen. And what it shows is for each frequency of sound that appears in this song, how much energy the bird has put into that particular frequency in the sound. And so what ...
[SOUND OF BYERS TALKING UNDER]
COHEN: After years of research, Byers has made a significant discovery: the male birds who have the most offspring almost always stay on a single frequency or pitch when they're singing the mate attraction song.
BYERS: So if it's a high pitch, they're singing it at exactly that same high pitch time after time after time. Some birds vary it a little bit when they sing a series of songs of the same type. The first one's high, the second one's maybe a little lower, and they're going high, low, high, low, high, high, low, and not keeping it consistent from song to song. Those birds have lower reproductive success.
COHEN: These are preliminary results. Byers will spend the winter looking at other aspects of these songs such as whether each syllable is equal in length to every other syllable. Such detailed analysis seems daunting, but Byers says understanding bird songs is worth it.
BYERS: They're among the most elaborate and complex vocalizations in the animal world, including humans, and just as we acquire language, songbirds acquire their songs. They're not born knowing them; they have to learn them.
COHEN: How the songs develop and evolve is the bigger question that drives Byers' work. The quest draws him close to the small details of bird life.
[SOUND OF TAPPING AND FOREST]
COHEN: Back in June, Byers pointed out a perfectly crafted nest cradled in the crook of a branch. Inside were two scrawny, just hatched nestlings, their heads as big as their bellies, naked, featherless.
BYERS: They are strange looking creatures and it's hard to believe that just about 12 days from now they'll have all their feathers and they'll be able to fly out of the nest. They grow so fast.
COHEN: Even after years of research Byers is still in awe of the chestnut-sided warbler. Now that he has shown that the males who father the most offspring sing the most consistently, he wants to turn to the females. Can females discriminate differences in singing style? And if so, do they base their choice of mates on those differences? Perhaps it's female choice which is shaping the evolution of the elaborate singing of the chestnut-sided warbler. For Living On Earth, I'm Nancy Cohen in Savoy, Massachusetts.
[MUSIC: Jewel Akens, "The Birds and the Bees", REMEMBERING 1966 (Dominion - 2001)]
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