Sounding Out the Dawn Chorus
The dawn chorus is one of the most conspicuous vocal behaviors of
birds, and one of the least understood. Near sunrise, birds often sing
more loudly and vigorously than they do at other times of the day.
Recent studies have suggested that these intense bouts of song may help
male birds exchange information about their social standing. However,
because so many birds sing at once, sorting out these vocal
interactions has been difficult using traditional techniques such as
observation and simple audio recordings.
Taking a new approach, Lab of Ornithology researchers John Burt and
Sandra Vehrencamp used microphone arrays at a field site in Costa Rica
to record the dawn chorus from 13 locations simultaneously on the
territories of four male Banded Wrens.
Using computer software and
techniques developed by the Lab's Bioacoustics Research Program staff,
they later pinpointed the locations of singing wrens based on how long
it took the sound to travel to different microphones. Song by song,
they analyzed spectrograms of the wrens' vocalizations.
They found that
many Banded Wrens were alternately directing their songs to specific
neighbors and responding to one another--the first evidence that birds
communicate in a networklike fashion during the dawn chorus. The
results were published in Animal Communication Networks (Cambridge University Press).
Banded
Wrens are tropical relatives of Carolina Wrens that inhabit deciduous
forests along the Pacific slope of Central America. During the dawn
chorus, males perch high in trees and sing an assortment of songs based
on a repertoire of 15 to 30 song types. Previous work by Vehrencamp and
Laura Molles, a former graduate student, showed that in territorial
disputes, a male signals his aggressive intentions by singing the same
song type as his neighbor. He switches to a non-shared song type if he
wants to back down.
"In the field, we could hear Banded Wrens matching one another's
songs," Vehrencamp said. "Sometimes we could hear song types rippling
through the population." The analyses confirmed high rates of song
matching, suggesting that competitive interactions are particularly
intense during the dawn chorus. Furthermore, males used different song
types to match different neighbors in rapid succession, evidence that
they were communicating in a network, rather than broadcasting songs
indiscriminately.
The
dawn chorus is a complex phenomenon, and most likely it functions in
more than one way, not just in social signaling. Burt and Vehrencamp's
contribution is twofold. They've shown that Banded Wrens are listening
and responding to one another, sending out purposeful and dynamic
messages to multiple neighbors. And in the process of recording and
transcribing the chorus, they've introduced some innovative techniques
that can be used to gain further insights about why birds sing at dawn.
--Miyoko Chu and Kristin Sargianis
This article was originally published in BirdScope, Spring 2004.
Related article:
Banded Wren Vocal Communication: Song matching and repertoire matching among Banded Wrens.