[TN-Bird] Adaptation in bird song
- From: "Charles P. Nicholson" <cpnichol@xxxxxxxxxxxxx>
- To: tn-bird@xxxxxxxxxxxxx
- Date: Mon, 18 May 2009 17:43:44 -0700 (PDT)
Students of bird song will enjoy the following article from the American
Association for the Advancement of Science's ScienceNow reports. It discusses
the work of the late Luis Baptista, one of the leading figures in the study of
bird song in this company. I had the privilege of meeting him and discussing
his work at AOU meetings years ago.
It has been known for years that the density of vegetation affects sound
transmission, and that lower frequency sounds transmit better through dense
vegetation than do high frequency sounds. The study described here is exciting
documentation of birds changing their song in response to a change in their
habitat. These coastal populations of White-crowned Sparrows are non-migratory
and Baptista mapped many discrete local dialects years ago.
Chuck Nicholson
Norris, TN
When Its Environment Changes, So Does a Sparrow's Tune
By Elsa Youngsteadt
ScienceNOW Daily News
15 May 2009
The year was 1970. Simon and Garfunkel topped the charts, floppy disks were
brand-new, and California white-crowned sparrows (Zonotrichia leucophrys)
sang fast machine-gun trills. Just a few decades later, the sparrows
sing noticeably slower songs, and a new study reveals the reason. The
birds' habitat has gotten scrubbier, and their melodies have evolved to
better penetrate the thickets.
Ecologists have argued for decades
that habitat influences the evolution of bird song. Slow songs and
low-pitched sounds transmit better through dense vegetation, whereas
high notes carry farther in open environments. And overall, grassland
birds do have faster, shriller songs than those from leafy surroundings
have. But researchers discovered this by comparing modern populations
or species from different habitats. They didn't know how long it would
take a species' song to adapt to a new environment.
Now they do, thanks to a collection of historical recordings of
white-crowned sparrows. Evolutionary ecologist Elizabeth Derryberry,
now of Louisiana State University Museum of Natural Science in Baton
Rouge, knew that, beginning in the late 1960s, renowned ornithologist
Luis Baptista had spent decades studying the birds' chattery, buzzy
songs. But she couldn't find his tapes. Finally, in 2003, 3 years after
Baptista's death, Derryberry tracked down the sparrow songs in the
professor's old office at the California Academy of Sciences in San
Francisco. Among the boxes and reprints, she uncovered the songs of 170
male sparrows from 15 locations on the West Coast.
Thirty-five years after Baptista's visits, Derryberry embarked on a
monthlong road trip to the same sites. She found that today's
white-crowned sparrows sing the same basic song as their ancestors did,
but the trill at the end is lower pitched and noticeably slower--on average
only 10.3 trills per second instead of the historical
11.8. That difference was big enough for the sparrows to notice: The
birds reacted more strongly to recordings of modern songs than they did
to old ones, with males moving toward the sound and females doing a
little dance. Derryberry wondered if a more densely vegetated habitat
might be responsible for the decelerating trill.
Finding out
meant tracking down another set of historical data: aerial photographs
of the birds' habitats. Derryberry reports in the July issue of The American
Naturalist
that she found historic and modern photos for five of the 15 recording
sites, and they showed that the birds' habitat had gone from being
mostly grassy, with about 11% scrub, to 26% scrub. The slower modern
songs would better penetrate the new scrubby habitat, which Derryberry
attributes to reduced livestock grazing.
That bird song can adapt so rapidly to habitat change is "an extremely
exciting finding," says Manuel Leal, an ecologist at Duke University in
Durham, North Carolina. Ecologist Matthew Betts of Oregon State
University in Corvallis adds that "it's something people have expected
should happen, but the data really haven't been there" until now.
Whether the changes in the sparrows' songs were cultural or genetic is
still an open question. Betts also wonders whether other bird species
would adjust their songs as quickly. These questions may not be out of
reach. It took Derryberry 6 years to assemble her epic historical data
set, but many more people are recording bird song now than 30 years
ago, Betts says. He expects to see a bloom of similar studies.
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