[birdky] Fwd: Something for the bird line
- From: DAWester2@xxxxxxx
- To: birdky@xxxxxxxxxxxxx
- Date: Tue, 13 Sep 2011 09:55:44 -0400 (EDT)
I thought some of you would like this HB trivia.
Doris Jessamine Co
____________________________________
Hummingbirdsâ?? Wings Sound Mating Call
September 12, 2011
Though famous for their mid-air hovering during hunting, tiny hummingbirds
have another trait that is literally telltale: males of some hummingbird
species generate loud sounds with their tail feathers while courting
females.
Now, for the first time, the cause of these sounds has been identified: a
paper published in _Science _
(http://www.google.com/url?sa=t&source=web&cd=1&ved
CgQFjAA&url=http://www.sciencemag.org/&ei=ggpuTqLVIYnRrQfT--TKBQ&usgAFQjCNEr-3hanKyuPSLBxYUdVCMTeRm2hA&sig2=JLIlZlNRRNpwWSFQEjqJfQ)
by
Christopher Clark of _Yale Univ._ (http://www.yale.edu/) reveals that air
flowing
past the tail feathers of a male hummingbird makes his tail feathers
flutter and thereby generate fluttering sounds.
Male hummingbirds only produce fluttering sounds during their elaborate
courtship rituals. Typically, during such a display, a male hummingbird will
climb into the air five to 40 meters, and then quickly dive-bomb down past
a perched female; when the courting male bird reaches the lowest point of
his dive, he rapidly spreads and then closes his tail feathers. This
spreading exposes the tail features to air, which causes them to flutter and
generate sound.
Clark's research, which he began as a graduate student at the _Univ. of
California at Berkeley_
(http://www.google.com/url?sa=t&source=web&cd=4&ved
DcQFjAD&url=http://berkeley.edu/&ei=cgpuTqLUAcnirAeU9fGmBQ&usg¯QjCNEakxDthi
X3fRomTrKtr_DEYAYM0Q&sig2=Sk-iYVvK1HRh-tOvekFriQ) , shows that the males of
each hummingbird species have their own signature sound--largely
determined by whether and how the fluttering frequencies of its different tail
feathers interact with one another and blend together. For example, the
fluttering of one of a hummingbird's tail feathers may cause a neighboring
feather
to flutter, similar to the way that the vibrations of one tuning fork may
cause another nearby tuning fork to similarly vibrate.
When interacting in this way, a hummingbird's neighboring fluttering tail
feathers produce a sound that is louderâ??usually about 12 decibels louderâ??
than would the two tail feathers fluttering independently of one another.
What's more, neighboring feathers that flutter at the same frequency produce
a different tonal sound than neighboring tail feathers that flutter at
different frequencies.
Other factors, such as the size, shape, mass and stiffness of the
hummingbird's feathers, also help determine the tone of each species'
particular
sound. "The sounds that hummingbird feathers can make are more varied than I
expected," says Clark.
When diving during their courtship dances, male hummingbirds exceed their
usual flight speedsâ??with birds of at least some hummingbird species
reaching their possible maximum speeds during dives. Also, the faster a male
hummingbird dives, the louder his fluttering sounds become.
In addition to diving during courtship rituals, a male hummingbird may
also brandish showy ornaments and produce sounds from other feathers besides
his tail feathers. Females choose a mate from among available males.
Clark is unsure why male hummingbirds have evolved an ability to generate
sounds with their tail feathers during courtship. But he offers two
explanatory hypotheses.
According to the first hypothesis, it could be that female hummingbirds
prefer to mate with the best flyers because of their relatively high fitness
and that females use the volume of a suitor's tail feathers as a proxy for
his fitness. If this were the case, perhaps males dive fast in order to
increase the volume of their fluttering and thereby appeal to volume seeking
female hummingbirds. The loudest males would thereby gain a selective
advantage and be favored by evolution.
Clark notes that common sense says fluttering sounds generated by actual
flight would provide a more accurate proxy of a bird's flying prowess than
would vocal signals generated by mechanisms unrelated to flight.
According to Clark's second hypothesis, it could be that the sound of the
hummingbirds' tail feathers developed incidentally to the birds' flying
abilities without such sound serving any particular purpose--similar to the
way that the sound of peoples' footsteps apparently developed incidentally to
the ability to walk without such sounds serving any particular purpose. At
the same time, it could be that female hummingbirds prefer the loudest
males for reasons not related to their flying prowess. If this were the case,
the loudest males would still have a selective advantage and would be
favored by evolution.
"This work is an excellent example of the use of physical approaches to
understand the function of biological structures, and it reveals
aerodynamic--rather than vocalized--signaling during courtship," says William
Zamer,
who directs NSF's Ecological and Evolutionary Physiology program. "It is
significant that the diversity of feather structures in these hummingbirds may
result from sexual selection."
Clark analyzed the fluttering sounds of hummingbird feathers by measuring
the fluttering feathers with a Scanning Laser Doppler Vibrometerâ??an
instrument that is used to measure the vibrations of a surfaceâ??and by
viewing high
speed videos of the tail feathers of hummingbirds in a wind tunnel.
Clark notes that the aluminum wings of airplanes may also flutter as air
flows over them, and in the absence of engineering controls, may even break
because of such fluttering. Presumably, hummingbird feathers do not break
when they flutter because they are less brittle than aluminum airplane
wings, and so can bend more without being damaged.
The study was co-authored by Damian Elias, of the Univ. of California at
Berkeley. Elias contributed expertise and resources, including the Doppler
vibrometer to measure the vibration of the hummingbird feathers.
Source: NSF
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