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Dawn Chorus: Can bird song identify environmental health?

Karl S. Berg is currently a Ph.D. student in the Department of Neurobiology and Behavior at Cornell University

The alarm clock just blared and you bury your head under the pillow. But not everyone goes back to sleep -- early risers are greeted by the “dawn chorus”. They enjoy the first birdsongs of morning and most don’t wonder about its ecological significance. But former STAR fellow Karl S. Berg did wonder and his research has produced significant results of use for environmental monitoring.

Bird populations have long been viewed as “canaries in the coal mine” for indicating changes in environmental health. As EPA’s Report on the Environment states, “changes in bird populations reflect changes in landscape and habitat, food availability and quality, toxic exposure, and climate.” Annual bird counts to document population changes among songbirds are conducted by the North American Breeding Bird Survey (BBS), in which a cadre of experienced birders documents their auditory and visual sightings. The BBS is one of only two indicators of biological diversity chosen for the EPA’s Report on the Environment (the other examines fish). In the BBS, an observer follows an assigned route, stopping for 3 minutes at designated points and recording all birds heard or seen near each stop. Observers start one-half hour before local sunrise and each route takes about 5 hours to complete. When the species’ calls are mainly at dawn, this is a severe limitation because these birds would be undercounted. Improved methods for monitoring bird populations are needed to better understand how they are changing and why.

For those who wake before the sun, it’s no secret that birds are also early risers. But why birds sing so much in the morning has puzzled scientists for a long time. The first pattern noted by 19th century naturalists was that different species began singing at different times each day, but more than a century later, no one had been able to explain why. Karl’s master’s thesis project, supported by an EPA STAR fellowship, set out to change this situation.

In his quest to understand the “dawn chorus;” i.e., why different bird species chime in at different times, Karl chose the tropical forests of Ecuador as his research site. Tropical forests are the most threatened terrestrial ecosystems on Earth and have large and diverse bird populations. As more forests are destroyed, many changes take place in the remaining forests that ultimately end in plant and animal extinctions. Often no one knows why. One immediate change is the quantity and quality of forest light. Karl’s study showed that common communicative and reproductive behaviors of forest birds are synchronized or have co-evolved with seemingly tiny changes in forest light.

Dagua Thrush (Turdus daguae)

Previous research had focused on bird communities in Northern Europe and North America, usually involving 20 or so species. Scientists found that as spring approached and days lengthened, a hormonal cascade was unleashed that resulted in reproductive activities including song. Light was an obvious cue for birds to start singing each spring but it still could not explain why song timing for various species should differ on any one day. It was as if birds were somehow coordinating their daily use of the airways like radio stations or cell phone providers.

Karl chose to look for the control mechanism in a tropical rainforest of Ecuador where hundreds of bird species occur together. Because it straddles the equator, day length in the forest changes by only a few minutes throughout the year. Karl and his wife spent several months trudging up muddy, forested mountains at 4:00 AM to make over 100 hours of recordings synchronized with twilight to determine if the birds had a singing schedule (also called “bio-acoustical” monitoring). They also measured meteorological information and tracked how high in the forest the birds tended to feed. Back at Florida International University, Karl identified 130 bird species from the recordings and logged the times of 25,000 songs. A colleague at the Louisiana Museum of Natural Science provided eye and body sizes for each species from museum specimens. Karl’s research showed that tropical birds began to sing only when they saw light. Big-eyed birds that foraged high in the forest canopy sang earlier. The late risers were birds with small eyes in the dark, dense underbrush. The control mechanism then, was a combination of ecological and morphological traits synchronized with an atmospheric one.

Comparing how bird species differ can provide clues to how song evolved and how it functions today. But birdsong is a complex trait. Like human speech, song starts in the brain but is influenced in many ways by the environment. To find a general explanation that could explain the collective, neurological behavior of all the birds in your backyard - or in one of the most diverse forests on earth - was a daunting task. Karl published the findings in the Proceedings of the Royal Society of London and the work was subsequently featured by the New York Times, Science, Nature, Smithsonian, Audubon, two radio stations, and Wikipedia.

This research is a vital link that provides the data showing why automated, bio-acoustical monitoring would enhance the BBS. Imagine a network of simple microphones and recording devices deployed along a BBS route, collecting data over the breeding season rather than during a 3-minute stop. Such technology is well within today’s technical and fiscal capabilities and would provide significantly more cost-effective data collection for scientific analyses. Automated bio-acoustical monitoring, supplemented by the sophisticated understanding of birdsong timing revealed in this research, could help EPA and others better understand our changing environment.

Karl Berg can be contacted at: ksb39@cornell.edu

To learn more about this STAR fellowship research:
http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/7098/report/0

EPA’s Report on the Environment: http://www.epa.gov/ROEindicators/

Listen to the chirping of the dawn chorus in Ecuador. (1:00 minute, 5.03 MB, WAV)

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